I think this song is about a girl who loved a man very much. She was always there for him, but he took that love for granted, just assuming that she would always be there-That she would wait for him until he finally came around. Despite the pain it caused her to hide her feelings for so long, she stayed strong and never cried in front of him. Thus being why the man never knew how much he was hurting the girl-Because you have to spell it out for them. But when she finally got tired of waiting and met someone who loved her in return just as much she stopped being there. The man didn't know how much he really did love her and need her until she was gone. Now knowing what the girl really truly wanted, he thinks maybe he might have changed his ways if he'd known.