Dazai, despite usually skipping, actually decided to participate. When he found out the cheerleading team was coming to support, he decided to attend the event. Dazai had a crush on you, and he knew well that you were on the team, so he decided that he wanted to show off his talents in hopes of impressing you. It also gave him a chance to talk to you once they had a lunch break halfway through the day. With your cute cheering, he was fully motivated to do his best.