Sterek being best childhood friends and scarily codependent to the point that it's normal to them but anyone else thinks it's weird.
Case in point Derek starts dating Jennifer and immediately she points this out and tells Derek that Stiles is weirdly obsessed with him. Derek doesn't believe her even after two months of her saying it he just blows her off until eventually he wakes up in the middle of the night during a sleepover at Stiles' and he just hears Stiles repeatedly whispering "mine, mine, mine, all mine, no one else's, not Jennifer's, just all mine...." directly into his ear(they fell asleep watching tv and started cuddling while sleeping).
He realizes two things then, 1.) Jennifer may have been right about the obsession thing, and 2.) he's really fucking into it.
So into it that he immediately dumps Jennifer but purposely doesn't tell Stiles about it. He starts pretending to sleep just so he can hear Stiles whispering, and makes up fake dates with Jennifer just to see the look on Stiles face.
He purposely drones on and on about her and all the dates they'd been on and how many times they kissed and how touchy feely she was, until Stiles is practically foaming at the mouth.
He pushes it further and further every time, just waiting to see Stiles reaction ,until about a month later he starts telling Stiles about his and Jennifer's first time together.
He's not even a quarter of the way through this imaginary story before Stiles absolutely jumps him.
They get together and now Stiles will just start playing with Derek's hair and whispering "mine, mine, mine.." at random intervals and Derek absolutely loves it.