Why do we transplant a perfectly normal organ? The organ itself has potential to function, it's only that we can't make use of it because we're either dead or too sick to continue.
Happiness, according to me, is the same thing. We maybe be broken and done for on the outside, but still have reserves of happiness inside of us. They don't have to rot with us. Instead, they can very well be transplanted to people who are really in need of it. People who want it. People who deserve it.
Sometimes, you don't make people happy. You transplant happiness into them. And live on inside them forever.