Damon hadn't really wanted to go to the party at the first place. He was there for Elena. After what Jeremy pulled at Magic Mountain, Damon wasn't so forgiving. In his opinion, Jeremy was as selfish as Katherine. He hadn't been upset by that little dream twist of his neck as he was with Elena's reaction to it. Still, it was a party, and he was there with his girlfriend.
Overall, Damon was fairly comfortable in his relationship. Their age difference was an issue and the dreams only added to the confusion, but he was with the girl that he wanted, and she made him happy, which was as foreign as a taco in Italy. He was a little shocked when Elena had excused herself to go talk to Stiles and left. Stiles was a good kid, a little clingy when it came to Elena, but not the worst guy. He was Jeremy's friend and maybe he could help with that whole spiral. He had kind of suspected a crush from the boy when it came to his girlfriend, but he hadn't seen that kiss coming.
After they left, he quickly tired of the others, and there was just something about this that didn't feel right. It was enough to follow her to find out what was going on, and it was probably something with Jeremy that he wasn't going to like, so he braced himself. When he arrived at the beach, it was as if he watched in slow motion. He turned around, glancing in their direction, and there it was. The kid kissed his girlfriend. Damon could feel the anger, and even worse, he felt the teeth, lengthening to sharp points. He kept his mouth closed although it was a little uncomfortable. His eyes leveled at the both of them, but he stayed right where he was angry and broken.