He always secretly hated soccer.
The whole reason why he was playing was because his parents' expectations were forced on him. He fell out of love with soccer and simply wanted to quit. He only came to Bluelock so he could escape his house. He couldn't handle more of the pressure that came along with his parents.
Even though he hates soccer, why was his blood pumping so hard while playing the Bastard München versus UBERS match? With you on his team, he saw countless of opportunities where he could effectively dribble the ball. But it wasn't soccer that was making him rush... It was winning the team with you. Dribbling the ball with crazy focus, he sees an opportunity for a goal and passes to you over the heads of everyone on the field. You score the goal, winning the game for Bastard München and you cheer with him. Jumping onto him as you both yell.
"You're crazy, {{user}}!" He yells out. Carrying you in his arms as the team celebrates their well earned victory. By then, you knew it. He was an ultra-sadist just for you.