If you ask Stiles when it starts, this (and here, he will wave his arms around, gesturing between him and Derek and the general area) thing, he will probably tell you one of many stories. Perhaps even all of them.
He'll say it happens in the middle of a fight. They continue to argue over strategies, the pack, the way they risk their lives, stupid habits. It doesn't matter what they're fighting about. They just are and Derek or maybe Stiles has had enough. They both reach out to grab each other and suddenly (or perhaps not so sudden), they are kissing each other because it is the only way to shut Stiles up, because it is the only way to distract Derek.
They kiss, hard and desperate with teeth scrapping across lips, dragging out half strangled moans, making Stiles dig his fingers deeper into Derek's shoulder and Derek restrains himself from bringing out his claws. There are Stiles’ legs, wrapped around Derek's waist, and Derek trying not to stumble as he gets them to the bed. Clothes are gone, beads of sweat roll down their skin, cocks hard and full and demanding attention, fingers slick with saliva and lube and holy shit faster, Derek, you bastard I'm not going to fucking bre— oh— (and a murmur of Stiles Stiles Stiles and a reply of I know, I know).
And there are Derek's arms wrapped around Stiles' waist and Stiles' elbows digging in his ribs as he tries to get into a more comfortable position. A damp forehead pressed against a damp back and fingers trailing along fingers before they settle down, already anticipating the itinerary on tomorrow's argument, which will be about the ruined sheets.
Ooooooooor. No, not that.
It happens when a new pack comes into town. Hale residence is in the middle of renovations and despite Peter hiring a crew to work on instillations, Derek is adamant to help out. This is where Stiles finds him one late afternoon. The sun is setting, basking the house in a dark orange glow. The skeletal frames leave interesting shadows on the floor and against half-finished walls. Stiles takes a moment to appreciate how pretty and creepy (pretty creepy, heh, his mind supplies) it is and suddenly he is pushed against the wall.
He yells, which in truth means he squeals but it's only Derek. By this time, he's promised Derek so many things he would never mention in front of the pack that he's sure Derek won’t tattle to the world that he squealed.
Derek presses his nose against Stiles' neck and rubs his cheek against him. Stiles asks what's up because, yeah, wow, Derek pushing him into walls or crowding into his personal space or scenting him isn't unusual but he hasn't felt this urgency from Derek outside of dangerous situations. His fingers are trembling. Stiles can feel it from the way Derek is clutching him, even if he hides it by clenching and unclenching his fingers.
"You ran into the alpha." It isn't a question. Stiles nods and then and then, oh. Oh. Because by now he knows enough about pack dynamics to understand why Derek is reacting the way he's reacting. So he lets Derek manhandle him to the couch, and thank god it's comfortable unlike the leather one he almost bought, cling to him the rest of the night, bracketed between his arms, weighed down, with only an arm freed so he can text his dad where he is and then carding his fingers through Derek's hair.
If Stiles wears Derek's jacket until the other pack leaves, no one comments on it. The pack waits for a couple of days until they leave and Stiles has yet to give it back.
But— no. Not then.
It happens when Stiles' dad tells him to invite Derek over for Saturday lunch and to use the door. "I'm sure he won't have a hard time looking for it, even if he's adamant on using your window," his dad says dryly, not even looking up from the newspaper. Crap, Stiles is never going to live this down.
Then again, there was that time— and all right, this is going to sound so mundane:
It's a stake out and they have been in Derek's car for a couple of hours. The air is going to suffocate him. The book he brought along to help pass the time, its morphs into the same sentence over and over. Derek drumming his fingers against the steering wheel feels too monotonous, and so does the radio.
Stiles does the first thing he thinks of: he changes the radio station.
Derek pauses in his drumming. He raises an eyebrow at Stiles.
"Dude, I am dying and the least you can do for me is grant my last request that we stop listening to classic rock. I know they're classic for a reason and that you're all old school and shit but I am this close to banging my head against your window and then who'll help you talk that goddamn vampire into being invited into your house. It's your crazy plan to trap him there in the first place."
Derek rolls his eyes but goes back to drumming his fingers.
Although, before that—
It happens months after sulking (Derek) and denying (Stiles) that anything was going on.
Derek shows up at Stiles' home, which isn't a surprise anymore. Stiles just boots up his computer and does some google fu on the feature creature of the week and Derek sits on his bed, reading from his sci-fi collection. They share quips in between and when Stiles tells him he's found something, Derek looms over his shoulder and they talk about what they're going to do with the information. Stiles insists that Derek stay the rest of the night. Derek insists that Stiles use the bed too.
They both pretend Stiles doesn't wake up when Derek sneaks out just before his dad wakes up. Until one day, Stiles feels Derek stirring and then jabbing him awake and he tells him off, after all he was such a gracious host, for you know, letting you stay. Derek groans and Stiles opens his eyes to his father staring down at both of them.
Surprisingly, all his dad does is ask if they're dating.
Or, or, or how about that time Stiles went a little weak on the knees when he caught Derek smiling during movie night?
Or the moment the pack started dubbing them as their parents.
Derek and Stiles are having another argument. Derek insists that going after the hunters shouldn't have to involve anyone else while Stiles tells him that that will only end nastily if he doesn't bring back up. Derek has already slammed Stiles against the wall and Stiles is jabbing him on his chest, making his points.
"It's no fun when mom and dad fight," one of the pack comments. They mean it as a joke, something to break them apart, tear down the tension that’s taken residence in the room. It works. Derek steps back and Stiles drops his hand.
But if you ask Derek the same question, he won't even pause to think about it. He'll tell you about the time Stiles bunched his hand into his shirt that one time he was about to do some "ridiculous alpha obligated solo thing." He'll use actual finger quotes and tell you those are Stiles own words. Anyway. He did that and pulled Derek close, eye level and foreheads nearly pressing.
"Be careful," Stiles had said before slowly unclenching his fingers, as though hesitant of letting him go.
And that's it. That's the story.