I watched "Just my Luck" with my daughter; and as a single father who plays a little guitar it was great to be able to play the song for her.
The meaning is simple. When I'm not with her the "world is such a lonely place" she is the "one that puts that smile on my face"!
When we are together, "it makes me fell stronger" and with out her " the days are rough and an hour feels much longer"