The thing rubs all over Stiles, getting its scent all over him, and the kid looks like Christmas' come early. It's stupid. The ball of fur is stupid. And Stiles is supid for picking it up.
Derek growls a bit. The cat looks at him for maybe a second and a half (how does a cat manage to look judgemental, anyway?), before going back to climbing all over Stiles as if it was the sole pupose for its inane existence.
"Derek, man, please. I really can't keep it home. Dad's allergic to cats."
"So go leave it where you found it."
"Out in the streets? Derek, it's a kitten. As in a tiny baby cat. It's a baby, Derek, it could as well be a baby dog... Wolf. Werewolf. How would you feel if somebody left a baby werewolf out in the streets where it'd surely die?"
There're so many holes in that logic that Derek ultimately feels it would be a dick move to start pointing them out. Stiles only falls back on these kind of less brilliant trains of thought when he's agitated or the tiniest bit desperate. It reminds him that no matter how brilliant the teen is, in the end he is just that: a teen.
The kitten's pink little tongue escapes from its mouth and settles comfortably on one of Stiles's fingers, where it licks away, diligently. Stiles melts.
"You better look after it. Because I sure as hell am not."
... Derek fucking caves. Because a simply happy Stiles is harder to come by than most people would expect and underneath all of his many issues and the protective layers of brooding he's encased himself within, he's still just a decent guy who wants the members of his pack as happy as he can have them.
The thing meows its approval at him. Stiles swoons.
He may growl a little more in petty jealousy.
But, reallly, who gives a damn?