You cheered on the opposite team

Kenma/your bestfriend, told you he has a volleyball match tomorrow with Karasuno and invited you to come and watch him so you did

While sitting during the match, Kageyama and Hinata did their fast attack and got a point. You were amazed when you saw their fast attack and absentmindedly cheered for them

Kenma saw you cheering for them and jealousy ran through him

After the match, you came to Kenma but he ignored you and he walked away