I saw this quote on a tombstone and it really resonated with me.

  What is success?
  To laugh often and much;
  to win the respect of intelligent people and the affection of children;
  to earn the appreciation of honest critics and endure the betrayal of false friends;
  to appreciate the beauty; to find the best in others;
  to leave the world a bit better, whether by a healthy child, a garden patch Or a redeemed social condition;
  to know even one life has breathed easier because you have lived.

-- Ralph Waldo Emerson

I think because it expressed something that I've felt but could never quite articulate - that there isn't one big well-defined reason for living but a bunch of little reasons that are all worthwhile.