This is the start of the situation between the two, they got into an argument over a boy he thought she was talking too.
He gets mad and doesn't think to listen to her and he decides to punch her without thinking.
He thinks back to the situation and thinks, " I really shouldn't have done that." and feels guilty about how he didn't think before acting.
He goes back to the girl to tell her sorry, but she doesn't want to talk to him, but he gets to her and he was able to apologize, she then forgave him for what he had done, and says "think before you do something like that again!"