He seen her with another guy it drove him crazy. He felt betrayed and worthy mostly angry insulted that a person would lead him on then act like what happen between them didn't happen like it didn't matter or mean anything.
He remained loyal to the wrong person once he moved on. She would try to take interest on his new romantic encounter. He would tell her to mind her business.
She would play her mind games saying she missed him when she wanted nothing to do with him. He'd ask her to be his gf but she said no but a week later would be in a relationship with someone else.
She didn't want a relationship with him. He was dating so he could get over her even though he wasn't ready. He needed time to heal.
He was one of many never meant to be the one in her life. His actions and behavior would get the best of him because he had never been in that situation before. Being a late bloomer was not helping but once he went through it he learned to deal with it.