Richard Dawkins, in his book The Selfish Gene, offers a good explanation of W.D. Hamilton's "selfish gene" theory.

He basically said that our genes are 'selfish'. More specifically, our genes want to reproduce themselves, and to continue to survive; behaviour that leads to genes' reproduction will happen.

An offshoot of this is seen in how people of differing genders approach relationships. Males, it is hypothesised, like to have short relationships with as many women as possible, spreading their genes around many people. A man who does this will have many offspring, and thus his genes will have a higher chance of surviving.

Women, conversely, are not going to have more than n offspring, since it takes a woman months to produce one baby, but it takes a man minutes. Women thus want long-term relationships, with the man providing support bringing up the child, to ensure maximum chance of survival.

Men can best propagate their genes by having many short relationships with their DNA getting spread around, and women can best propagate their genes by getting men involved in long-term relationships, providing support and childcare, ensuring their children have the highest chance of surviving.

Obviously, this is just a hypothesis. But is seems relevant.