Derek knows Stiles does this on purpose: wearing his shirts, stealing his socks, borrowing his sweaters--all so he'll smell like Derek, something he knows drives Derek up the wall.

So Derek really shouldn't be surprised when he arrives home after a run through the Preserve to find Stiles sitting on the couch and wearing his red thumb-hole sweater.

"What?" Stiles asks with a smirk that means he knows exactly what he's doing. A smirk that turns into a smug smile as soon as Derek pounces on him, gets him flat on the couch, and leans in to give him a kiss.