I've always wondered about this as well, and happiness seems like too simple an answer, but occam's razor would suggest it is the right one.
That said, a life's short insignificance is also what makes your life so significant - you only have a short time here. You have the opportunity to make a measurable difference in the lives of other humans, whether you like it or not, like so many humans have before you (Hitler, Mother Teresa, Jobs, Napoleon, Genghis Khan or Socrates). No matter how trivial you might consider the achievements of the people who's names we all know, they affected the lives of countless people. Some for the better, some for the worse.
humanity will continue without you, as will the universe, but a human's power amongst other humans is so great, it should be ones goal to make the lives of humans better, enriched, directly or indirectly, through your actions. To improve the lives of others through your actions - whatever that manifests as, is I think a good goal.
It drives me to do what I love, for the people I love, to manifest a world that I can love. There's too much wrong today, and I may be a straw in a straw pile, but eventually that camel's back did break! Or so the story goes.