The first time Dave realised he might be falling in love with Sebastian, they were tangled together on Sebastian's bed. Their shirts had vanished at some point over the course of the evening and Sebastian's mouth was in the process of making its mark on Dave's neck. Dave recognised that it was about time they broke apart and tried to cool down before they did something they'd regret tomorrow, but leaving Sebastian's embrace was the last thing he wanted to do. That in itself wasn't unusual, what was different was that the reason he didn't want to stop wasn't simply because he was a horny teenager embroiled in an incredibly hot make out session with his incredibly hot boyfriend. It was because, despite the fact that they'd spent the last half hour pressed together with their tongues deeply exploring each other's mouths, their hands memorising the shape of each other's bodies, their legs gloriously intertwined, it still felt as if they weren't close enough.
He wanted to hold Sebastian tighter, remove the final remaining layers of clothing that separated them, and connect their bodies completely. However, the more rational part of his brain was still functional enough to realise that he was a horny teenager as well, and his current state of arousal probably had some influence over his judgment, so instead he pulled Sebastian into one last lingering kiss and moved over to the other side of the bed, resolving to re-examine the idea when his head was clearer.
The first time Sebastian realised he might be falling in love with Dave, they were curled up on the couch in the Karofsky family home watching some movie that Dave loved and Sebastian tolerated because one of the main actors was hot (and because the way Dave geeked out over the film was weirdly adorable, but he'd never admit to that out loud). They were well aware that Dave's father was in the next room and they couldn't do any more than share a few chaste kisses, but Sebastian found that he didn't really mind. It slowly dawned on him that, in fact, he didn't want any more than that right now. He was perfectly content to spend the rest of the night like this, simply revelling in their closeness, surrounded by Dave's warmth, breathing in his scent. Dave's mere presence brought him a sense of completeness that he'd never experienced before. He wasn't sure whether to be happy or terrified, but he decided that if he was going to freak out over what these feelings might mean he could do it later. For now he was simply going to rest his head on Dave's shoulder and enjoy the moment.