Okay imagine Derek showing up to Stiles’ place all sweaty and bloody, because “I was attacked and it’s not safe to got to my place, because they know where I live” so Stiles let’s him in.
He let’s Derek use the shower and borrows him clothes. And of course he won’t let Derek choose them himself, because “You’re not taking my best clothes this time, no way… and I know which ones are big enough for you”.
So while Derek is in the shower Stiles goes through his closet to get him the clothes and he let’s himself in the bathroom (“Wha- STILES get the fuck out!!!”) to leave them on the toilet lid. (“Relax dude, it’s not like I haven’t seen a dick before” *a cheeky wink as Derek blushes*)
And after the shower Derek bursts into Stiles room.
“Stiles, what the hell??? I’m NOT putting these on!!!”
Stiles looks at naked Derek standing in the middle of his room with pink Hello Kitty boxers in his hand.
Derek watches as Stiles’ eyes move from Derek’s face to the boxers in his hand and then to his- … OH SHIT
Derek goes red and bursts back into the bathroom and showes back up minutes later - dressed this time - trying to avoid Stiles’ gaze which is, by the way, locked to Derek’s crotch.
And yes, he has the pink Hello Kitty boxers on.