This question is akin to "what is the meaning of life".
Each person has their own meaning. Women, when asked, will usually talk about having children, a family, someone of their own.
Men (I haven't asked men I believe) would talk about self-fulfillment, goals, family also, success.
These male viewpoints differ greatly from man to man, i.e. what will make my life full, what are my goals.
Remember we evolved as species by guaranteeing our survival in hostile environments, and guaranteeing our existence by procreation.
Therefore, the things that helped us get here as a species are the same motors that drive us today:
PROCREATION + SURVIVAL
Procreation is fairly straightforward. Not in vain do we produce millions of sperm each day. To use them.
Survival on the other hand is more complex. It can mean economic stability (usually through jobs) so as to be able to get healthier reproductive partners, personal health (which is why exercise and a healthy diet tend to make you happier through the release of endorphins), resistance to attackers, shelter, food, etc.
Then again our higher cortical brain also has it's share of issues, affecting this balance and putting other perceived items on our happiness agenda.
Just my 2 cents.