This is another sad song for an ended relationship. The saying 'You don't know what you have until it's gone' is the theme of this song.
Taking for granted the guy and your relationship together. Then finally when everything's ended it hurts because you know you gave up your chance for something that could've lasted forever. It still hurts when you finally meet each other because you know, you know it deep in your heart that that he could've been the one.