I didn’t write this, but I think it explains perfectly what the song is about. Or at least, how I feel right now.
This song is about falling in love with the person you can never be with. A love with consequence: a cat can never be with a mouse because no matter how they love each other, the cat will end up hurting the mouse. In the song, the man eventually ends up hurting the woman, and feels sorry for it. And he asks is he supposed to be happy when all he ever wanted came with a price. The price is the woman’s life or her own happiness.