When Derek and Cora leave town, Stiles figures out pretty quickly that there’s a stupid Derek-shaped hole inside his chest, right next to that damn darkness around his heart that Deaton had warned them about. Then, Derek invites him to the Atlantic Ocean, where a certain couple of werewolves have made their new home, and Stiles slowly comes to the realization that his definitely-more-than-just-a-crush might not be completely unrequited, after all.
"So this is what Stiles does. He lies in Scott’s bed and waits for Melissa to say she’s found someone to get it out of him, to cure him of the wrongness and the bad, and he dreams.
God, he dreams.
He dreams of fire and swollen bellies and that scene in Alien, of giving birth to jackals through his urethra, the whole horrific nine yards. His head is a terrible place to be, he can’t imagine his stomach is much better, why anyone would want to put a thing inside of it."
After Stiles gets possessed and de-possessed, he falls into a depression. It's a time in his life he's not proud of. He's unable to help his friends who seem to have no problem getting on with their lives and battling all evil that threatens Beacon Hills, with a smile on their faces. , He's unable to do all the big things with his life he was expected to do.
He knows there need to be some changes once he gets off of his anti-depressants: the sheriff and Derek think it would do him good to start working out, and since Stiles doesn't really have a good reason not to, he agrees.
In which Derek is a terrible life coach, but at the same time he's exactly what Stiles needs.