Scott- when he defends himself and takes control if the situation you can't help but feel proud and aroused.
Stiles- His humour in general just turns you on. Especially when he uses it to comfort you or cheer you up.
Derek- watching him work out, duh. Sometimes he puts in a special show just for you.
Jackson- When he gets arrogant and cocky before a big game. His sex drive is usually way up at that point, so, you can't really help it.
Isaac- the way he acts around people. How shy he is. Oh, and the smirk that looks so glorious on his face, especially just before he seals the deal as he goes down on you.