Mike Rosenberg explained the song's meaning: "In my mind the song has two meanings"; he said, "the first is quite literal as I wrote it after a break up and it is about letting her go. But then there is a bigger idea going on and is more about not really understanding and knowing what you have until it is gone and I think everyone can definitely relate to."
One of the things we humans do is habituate; we become used to things that are just there. The end result of this kind of habituation is that we tend to take for granted the things that we have and it does not mean that we'll miss it when it's gone. After all, getting used to something doesn't mean that we won't miss it terribly when it's gone.