My new character is a level 9 pact of the chain celestial warlock. I need ideas for a patron and what motivations a celestial would have for having a warlock in the first place?

I don't understand why people feel like they should understand the reasons why gods or patrons grant abilities to characters. 

The character has the option of being a farmer, or a merchant, or an adventurer; if adventurer, choices include Cleric, Monk, Warlock, etc. 

Does the patron have detailed goals for the character, or does the patron even notice individual Warlocks? Maybe the patron gets some incomprehensible benefit from having arrangements with Warlocks - like better seats at the next convocation, or more voting rights on the celestial council. Or one-upmanship vs other patrons. Regardless, my take is that it's none of the character's business.

That's how I play my Warlocks, anyway: the character has a backstory, but the character doesn't know the patron's backstory or motivations.

It could simply be your Patron's way of influencing events on the prime material plane in lots of locations rather than drawing attention to their objectives with their personnel engagement.  Why did Sherlock Holmes have the Baker Street Boys?  Or, if your Patron is a Great Old One, it's likely all about expendable resources.

I have two warlocks with Celestial patrons.  One was exploring as a child, got lost, and found a unicorn's glade in the forest.  The unicorn took him home to his parents and he went back a week later looking for the horsie.  After repeating this a couple of times the unicorn promised to come to him and teach him if he agreed not to explore the woods without his parents' permission.  There was the pact that carried through when he became an adult.

I'm the DM in a campaign where a player has a PC whose patron is a Pegasus.  He wanted a unicorn and I asked if I could switch it to a Pegasus for story reasons.  It's set in the Norse mythology and the Pegasus is grooming the PC to become the Pegasus's Valkyrie after she dies (the PC is a female) because the Pegasus doesn't have one and "needs" one.  The PC is a flower child and the Pegasus wants the PC to die in battle which the player doesn't know yet.

So the answer is you can come up with any creative and fun in story reason.  

I’d say a celestial has the same motivations as any being that creates warlocks. They have things they want done on the prime material plane, and they give powers to some people so they can have agents to get those things done. It’s a transactional relationship. I give you powers, you give me services. 

Well, i did a celestial patron that made the pact because his aasimar (son) that was supposed to be a hero, had fallen and drank himself to death.  The bargain was made to save the aasiamr's life, but instead of being "chosen" he was conscripted

The ways of the gods are inscrutable, and the ways of the celestials likewise.

All the Warlock needs to know is that they are to act righteously and resist evil.

What if there is no Patron?  I recently came up with an idea for an Aasimar Hexblade Warlock where their powers, and heritage, were actually the manifestation of their former powers as a Celestial who was reincarnated after being struck down in a great battle.  Kinda like how the Avatar from the Last Airbender/Legend of Korra has powers derived from the spirit of light that is fused within them and the past lives connected to it.

Your Patron is Nilbolg - The Unnamed goblin trickster deity who infects goblins with "Nilbogism".  The unnamed deity divided his divine nature into fragments to hide himself/herself from the wrath of Maglubiyet.  

Your character has either been infected with Nilbolgism or you are a child of a Nilbolg.  Either way, you have been gifted with a celestial spark.

You don't know what your patron's motivations are besides playing childish pranks on you.

Your character is from planescape. You found yourself in Elysium, a plane of good. You stayed long enough to get infected by the plane and never wanted to leave. Your friends forcibly removed you. You long to go back and spend the rest of your days there.

your patron is the plane itself. You feel you were pulled away because you were not yet worthy. So you warlock status is like unto a paladin. You strive to defend the week, fight evil, and when you are old, you will buy a wagon, load it with building materials and take the great road...going home to Elysium.

A solar has plucked you from "Wall of the faithless" and replaced your soul with the original soul of your current body, the solar is slowly decaying and claims you are needed to restore a long forgotten dead god and you two are the only creatures that can restore that god back to their former seat among the divine.

I'm making a Warlock with a Celestial patron right now, and having a lot of fun with it. I was inexperienced with Celestials so in my search I stumbled across the Shards, planetar servants of Selûne the Moonmaiden (who also has her own fun history).

So basically, my warlock's patron is Selûne, but she only ever communicates via her Shards. She and her family were killed by Selûne's evil sister Shar, but she was rescued and resurrected by a Shard, and now serves to protect and save the innocent under Selûne's guidance. (For some spice, the price for her resurrection was her memory of everything before her death--so she knows she was fatally injured but doesn't know how, why, to what extent, or what she lost in the process. The campaign is going to be one with horror elements, so I went a little more dramatic than usual.)

I made sure her resurrection (simply via the resurrect spell, which a Shard is perfectly capable of) didn't give her any buffs or nerfs the character wouldn't normally have, and it gave me a good reason for the amnesia I had already planned to give her. I've heard some discourse about how this subclass is basically a weaker cleric, but I'm enjoying the story anyway.

I'm playing a Celestial warlock in curse of strahd where we, as only two players, went with one main character and one a bit more passive. This led to me first rolling a ranger, then rolling for random background shenanigans and... Weird results later I made the warlock and to make it fit with my main character this is the story:

The main character was a monster/undead hunter subclass ranger who for various reasons hunted undead. In his travels he and a powerful wizard had heard rumors of a young man in a small village who seemed to be blessed in some way. It was rumored the young would get visions from the gods or something like this. Therefore, my character decided that it would be a great idea to perform a ritual which was intended to allow them to speak with someone in the celestial realm, because he needed some help finding someone.

So, they "convince" the youth to help them, sit him down in a ritual circle, mark him with a bunch of arcane and celestial runes and sigils and whatnot, perform the ritual aaaand it didn't go as planned. Instead of the boy being used as some kind of phone to call the celestial realm, they bound a celestial inside him. 

First of all, I used this as the basics for both the celestial warlock powers + the race which now was aasimar. Also I played him as a mix of the youth and the sometimes angry celestial (scourge aasimar felt best as a variant for the "celestial power rages through you" since he actually has one inside him). Part of the reason they travel together is because the main character felt bad and wants to undo it, but just cant figure out how. To make things more interesting I decided to play the youth more in the beginning but as their power grows, since it's basically because the celestial is taking over more and more, the soul of the young man is burning up. Each time he uses the scourge ability theres an anguished scream of the young mans soul as more and more of it burns away. This also means he becomes more and more focused on destroying evil and gets less and less caring about other living beings.

I felt this was really fun to play and worked well the way I did it in this group because it made sense to be more quiet. I also took the invocation so he never sleeps which gives him a creepy feel when everyone else sleeps and he just stands there. If I hadn't had two characters it would have been easy (and more effective) to actually play out the mix of personalities even more, sometimes the celestial, sometimes the young man. 

Of course, nothing lasts forever... Reincarnation and similar abilites help though! So, now.. He died two sessions ago. And I got reincarnation cast upon me. And I rolled on the list and apparently you can become a thiefling. (also before the reincarnation I decided it wouldn't make sense for the celestial to want this so I rolled a save for it but rolled a 1, he had inspiration though so I used it but rolled another 1... So in a short moment of desperation, the remains of the young mans soul accepted.). Anyways, because of this (and... secrets) their souls merged even more than before , with the young mans soul actually gaining the upper hand and is more or less leeching the powers from the celestial trapped inside him instead.

So, I'm playing a human/celestial/thiefling warlock which gets his powers from the celestial within. Also I dropped all my teeth. It's getting weird. :) But still fun!

I am playing a warlock - Pact of the Chain, whose Celestal patron is none other than Lulu, the hollyphant, who is still hoping to redeem her former friend and took on my char as help. Lulu found me because my character - a tiefling - is of the same bloodline as her friend... ;) Gives the DM all chances to send me on missions for Lulu.
Needless to say, my familiar also is a very tiny hollyphant ...

I have one Tiefling Celestial Warlock on the shelf who works in the plausible deniability department for a neutral good power.  When a guy walks into an encounter holy symbol aloft it gets a little hard to explain your way out if they fail.  Think of it like spies for Heaven.

I run a Celestial warlock who made their pact with the fallen angel Zariel (because fallen angels retain their powers after the fall). They are a Scourge Aasimar whose theme is light and darkness (lots of spell options for warlocks here) and uses the Gift of the Ever-Living Ones to maximise on bonus action self-healing.

Their reason for making the pact is a lover who died at the hands of an angel while part of a cult performing a ritual to summon Zariel for devilish warlock powers. When she realised who had killed her love, she completed the ritual and made the pact in her lover's place. From that day on, she has been sworn to advancing the ideals of the Nine Hells.

I have an idea for a Celestial Warlock I am trying to flesh out. He is a Tiefling from a rich and powerful crime family that is devoted to their infernal patron - still not sure if my character knows their criminal history or just thinks they are legitimate - although brutal Nobles.  His patron will be a celestial that is opposed to the family's infernal patron and his goal is to "corrupt" their offspring. 

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