James: You broke up with him because you thought he was in love with Lily.

Sirius: You broke up with him because he hexed some Slytherins for no reason and they were your friends. You got mad and said you can't be together anymore because he doesn't respect your house and your friends within it.

Remus: He broke up with you because he was insecure about himself and his werewolf problem.

Peter: He thought you had feelings for another person so he admitted he thought you deserved better than him and broke up with you.

Regulus: He broke up with you because he was getting in too deep with the death eaters so he wanted to protect you and keep you away from all that.

Lily: You broke up with her because she was adamant about hanging with Snape who was always awful to you and  hated you.

Marlene: She broke up with you because she thought that you were interested in another girl.