It all starts as a joke, really. Stiles probably doesn't remember but Derek sure as hell does.
It was the first movie night they did after they moved in together. Stiles was proud of the sofa he had picked and might have wanted to show off just a little bit. Their friends came over, there was pizza and movies waiting to be watched. Stiles had given a piece of paper and a pen to everyone so they could write down the title of a movie. All the movie titles went in a bowl and Stiles made Derek choose one, because they were the hosts, he said.
Nobody ever took the blame for writing down Dirty Dancing, the mystery has still to be solved. But they did watch the movie and it ended up being a nice evening full of laughter.
The first time Stiles said it was when Derek got up to grab more drinks from the kitchen and Stiles had called “don't forget the ice baby”. Derek could hear their friends laugh in the other room and he thought that 'baby' was better than 'dude', and didn't say anything.
It went on the whole evening, “baby” here and “baby” there. Stiles seemed to have plenty of fun and, thinking it would be only for a night, Derek let him go on with the pet name and the mocking smile that came with it.
The thing is, Stiles really likes his own jokes. He can laugh at something he said for days. Sometimes he tells the same story hundredth times because it was so hilarious and Derek is aware of the fact that anyone else doing this would be really annoying.
It didn't stop because Stiles was obviously convinced that it was the most funny thing ever.
Usually he gets tired of his jokes, but it seems like he got used to calling Derek like that and. Yeah. Derek wouldn't say he like it, because, duh. But he got used to it as well.
Derek wonders if Stiles, in the beginning, wasn't waiting for the moment Derek would say stop, that it was ridiculous, stupid or something. The first three weeks, it was like he was looking for an excuse to use a 'baby' every two hours, just to piss him off. “Baby did you check the mail today?” “Your pancakes are delicious, baby.” He always said it looking at Derek in the expectation of a reaction, but Derek used to Stiles's antics didn't gave him any.
A couple of weeks ago, Stiles said it at the grocery store. It was Saturday afternoon and there was a bit of a crowd there. They had parted each with a half of the list because they were in a hurry and when they were in line at the checkout, Stiles asked “Did you get the right flour this time, baby?” Derek is a grown men, but he felt himself blush at that. Blushing made him more embarrassed and for the first time he really wished Stiles would stop calling him like that.
Scott calls him 'baby' sometimes, too. Because, let's be honest, Scott is the only person other than Stiles to laugh for days at Stiles's jokes.
Cora doesn't comment on it, her eyebrows do. She and Derek are sitting on the couch watching a stupid movie because it's Sunday and that's a thing they do. Stile's, as usual, makes a noisy entrance when he comes back from Scott's. “Baby I'm home,” he calls. Cora's smile is saying a lot, there's a “really?”, with some “you guys are ridiculous” and a bit of “you're getting soft bro'”. Derek shrugs and goes back to the screen, waiting for Stiles to come and kiss him from the back of the couch, like he likes to do. That's a thing Derek really likes and for one of those almost upside-down kisses he'd let Stiles call him baby ten times a day.
Derek is glad with the fact that Stiles never did it when they're having sex. His multiple attempts at dirty talk are enough of a turn off without a 'baby' in each sentence.
There was that time when Stiles called him baby at his work. Stiles car had died that morning and since he needed it more than Derek he had taken Derek's for the day, giving him a ride to the library he worked at promising to be on time to pick him up. Right on time he was, pushing the door open he smiled and asked “you ready baby?”. Derek's coworker laughed at him for two weeks, calling him baby all the time. Thankfully she got tired of the joke. Unlike Stiles. Or Scott.
Derek has been happy to hear it once. Just once. It was the day he had to bring Stiles to the ER for a broken arm. In the waiting room Derek had freaked out but when a nurse came for Stiles, telling him they were ready to take care of him the younger had turned around, arm cradled against his chest and smiled softly. “It's okay baby, don't worry.” The nurse had smiled in an awe and Derek was relieved. He trusted Stiles and believed that everything would be okay.
On the day of their fifth anniversary Derek had to work while Stiles stayed in bed the all morning. It wasn't fair but it wasn't like they used to spend the day doing stupid couply things to celebrate (unlike Scott and Allison). Derek had made a reservation in a restaurant they'd choose together and after that the sex they would have wasn't going to be crazier than usual. In his car on they way to the library Derek put the radio on and laughed when he recognized Baby One More Time. It didn't take long before he realized he wasn't listening at the radio but at a CD made of songs with a fair amount of 'baby' in the lyrics or at least once in the title. He smiled, because Stiles wasn't a romantic at heart but he was stupid enough that his jokes could be mistaken for great love gestures.
It started as a joke but Derek is pretty sure Stiles keeps calling him baby for the simple reason that he likes it. And if he has to be completely honest, Derek might concede that he does too because somehow, Stiles calling him that shows that he, and only him, knows how much of a softie Derek is in the inside because he's the only one who's ever ventured that deep in Derek's soul. And if that's cheesy, Derek will gladly live on cheese the rest of his entire life.