There are many, but I think this might be my favorite quote from the book Eat, Pray, Love-
"All the sorrow and trouble of this world is caused by unhappy people. Not only in the big global Hitler-'n'- Stalin picture, but also on the smallest personal level. Even in my own life, I can see exactly where my episodes of unhappiness have brought suffering or distress or (at the very least) inconvenience to those around me. The search for contentment is, therefore, not merely a self-preserving and self-benefiting act, but also a generous gift to the world. Clearing out all your misery gets you out of the way. You cease being an obstacle, not only to yourself but to anyone else. Only then are you free to serve and enjoy other people."
- Elizabeth Gilbert
And isn't that the whole reason we're here? To serve and love one another?