Looking back, Derek should have known to recognize the signs that maybe Stiles didn't really return his feelings as Derek had originally thought and god, did he feel like an idiot.
Then again, he and Stiles did make out at Jackson's party that one time when she was drunk, and the fact that Stiles had woken up next to him with both of their clothes messed up and yet she didn't bat an eyelash should have been a sign for Derek that Stiles was not interested any further than a quick, drunken not-quite-all-the-way hook up.
Derek, just like everyone else, had been quite aware of Stiles's not so subtle crush on Lydia Martin for the longest time but Derek had thought that maybe she had gotten over that and moved on to Derek's welcoming arms.
Staring at the back of Stiles's dark, long hair in class, he sighs and wishes she'd glance at him, give him a sign that she returned his feelings because he was getting desperate. He didn't know exactly when these feelings had developed for Stiles, he just knew that it felt like they've been there for an eternity and every time he saw her, his heart wanted to escape from his chest and he could feel a jolt of warmth in his belly.
Maybe, he should have warded himself carefully against feelings, especially after what Kate had done to him.
He watches Stiles's hair fall to the side of her face as she leans to her right to whisper something to Scott and both laugh airily. On the left, Lydia looks at them expectantly and so Stiles leans in to whisper in Lydia's ear - her lips brushing softly against her strawberry blonde hair and Derek watches carefully the look the two share - the breathless smiles and Scott's knowing smirk.
Stiles looks away awkwardly and tries to focus to the front of the room and so does Derek, but he manages to catch Lydia's pale hand brushing lightly against Stiles's own.
Derek frowns.