In the video the man disappears in the end, he chooses to leave even if he thinks he will be nothing because they tried a long time but could not forgive each other. So he became "nothing" because staying wasn't helping staying was only haunting. He says what if I was nothing with out you, meaning maybe she was the only one really in love and he was just in it for a ride. That maybe the relationship wasn't real to him thus he is not really there just haunting her.