Because he wants to get with the female lead. In the end though, he feels super guilty about what he has done because of the effect it had on them and sets out to fix it. He does. 

Love Relationship - not exactly but almost because they never got into a relationship in the first place

  • Loving Never Forgetting
  • My Love Eundong

Hi! School - Love On a little bit? He successfully separates them (physically, by getting the ML to let the FL live with him - instead of with the ML - for her protection). He later does the right thing and lets her go back / forces himself to move on, even when she has amnesia and thinks she likes him and not the ML.