Someone once asked me what I regarded as the three most important requirements for happiness. My answer was:
A feeling that you have been honest with yourself and those around you; a feeling that you have done the best you could both in your personal life and in your work; and the ability to love others.
But there is another basic requirement, and I can’t understand now how I forgot it at the time: that is the feeling that you are, in some way, useful. Usefulness, whatever form it may take, is the price we should pay for the air we breathe and the food we eat and the privilege of being alive. And it is its own reward, as well, for it is the beginning of happiness, just as self-pity and withdrawal from the battle are the beginning of misery.
— Eleanor Roosevelt, You Learn By Living