The belief that unhappiness is selfless and happiness is selfish is misguided. It's more selfless to act happy. It takes energy, generosity, and discipline to be unfailingly lighthearted, yet everyone takes the happy person for granted. No one is careful of his feelings or tries to keep his spirits high. He seems self-sufficient; he becomes a cushion for others. And because happiness seems unforced, that person usually gets no credit.
People always think they know other people, but they don't. Not really. I mean, maybe they know things about them, like they won't eat doughnuts or they like action movies or whatever. But they don't know what their friends do in their rooms alone at night or what happened to them when they were kids or if they feel sad for no reason at all.