After Seth chose Anna, you decided to move on. Although you were so jealous.
Seth was happy, sort of. Until he realized that he and Anna are terribly similar and they never had a fight. Sometimes it was even a little boring for him without some fights. Anna loved the same comics, movies and others.
Seth remembered you. How you and him were constantly fighting over little things and when you beat him because he annoys you.
And so. At lunch, Seth finds Anna, you, and Luke all laughing about something or someone. You introduced them to your new boyfriend, Danny, and he cracks a joke about Seth's lunch. Everyone is laughing but Seth who is utterly confused as to what's funny.
Seth absolutely didn't like Danny. Everyone was saying how witty Danny was, which infuriated Seth a lot. Was he jealous of you? Very.