I don't know, I didn't think Moody was that possessive - to be honest I kind of thought Pearl was being a bad friend. They had already made plans to skip the dance and watch the movie instead. She convinced him to do both, and then was pretty obvious about only wanting to go the after party because Trip and Lexie were going to be there. I think Moody snapped at her then because he was hurt. He's been a good friend to her, and while she's obviously under no obligation to accept his friendship, they have been good friends up till the dance. But from his perspective, she basically blowing him off to spend time with his more popular siblings. I think many teens would be hurt by that.