Here's a nice quote from the book I'm currently reading:

-What is happiness?
- It is the possibility to fully make use of ones capacity.

Napoleon Bonaparte

(My translation from swedish, which was translated from english, which was translated from french)

While I don't fully agree with this quote it is fairly close to the truth. I think there are other aspects to happiness as well such as sharing your life with others. But anyway it's a nice quote.