this song is about taking love for granted, and then losing that love. They had "too much" love and "too much" time, so they figured that "tomorrow was promised" when it wasn't. It's like the philosophy that one should live like they're dying--they don't know what will happen tomorrow, so they should live life to the fullest.
It seems like this song is about someone who had too much time, took it for granted, and now has regrets, possibly because his love is gone.