I'll say the obvious - this is a song about unrequited or unobtainable love. But it clearly goes beyond just the "I love her, she doesn't love me" part. With the lines "Never had real love before," and "His love is all she knows," it seems that she has a boyfriend, but he doesn't really love her. Or at least not as much as Bruno Mars does.
Personally, I think it sounds like her boyfriend is the kind who says he "loves" her, then abuses her and makes her become so dependent on him that she can't leave him. I think the lines "Once again you're home alone, Tears running from your eyes," "Everyday, same old things, So you're still feelin' pain," and especially "All she knows is the pain, In the corner of an empty home, She's still comfortable," support this.
But then again, it could be that she has a father who is abusive, is never home, and is possibly a drunk. The lines about the empty home seem to support this. Though Bruno saying "But would that be selfish?" tilt it more towards the boyfriend theory.
Either way, this is such a beautiful song, one of my favourites. Bruno Mars is such a great singer, has wonderful songs, and I hope he'll keep writing and singing great songs like this.