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.
Gretchen RubinHappiness can seem like an abstract, transcendent notion, but in fact, I found that getting enough sleep (very important!!), getting exercise, not letting myself get too hungry, not letting myself get too cold (I'm a person who is always cold), made a big difference. Partly because I felt happier, partly because feeling physically comfortable makes it easier to keep other difficult happiness-boosting resolutions like biting my tongue.
Gretchen Rubin