I used to be of the opinion that it was human nature to procreate as often as possible. It is evolutionarily beneficial, I would argue, to spread your seed and produce the most copies of your genes. I found our natural instinct to be savage and selfish. Society was the reins that held back our instincts and admonished us. Marriage was an odd concept. Why would we remain with only one person when we should be out replicating our genes? My perspective on this changed greatly after reading Richard Dawkin's book, The God Delusion.
Where after all, does society come from? Does it benefit an organism to act selflessly by helping and protecting others?
Some would argue that modern society started out as mutual cooperation among people in an in-group. It's sort of like how monkeys clean each other. An organism will help another, with the expectation of being helped in return. There is also the benefit of strength in numbers. An organism is safer in a group. It has been observed that some organisms use charity to assert dominance over others in the group. By doing this, the organism is saying they can support themselves and have strength left to provide for others. In these types of societies, giving food to the dominant organism is considered a challenge to their leadership.
Charity makes us feel good. When we do something nice for others, we are rewarded with a happy feeling. That trait is evolutionarily beneficial for our species. It is my belief that the evolution of charity helped shape modern society.
But what about family? What does an organism have to gain through monogamy?
One of the biggest benefits that I can see, is that monogamy gives children two dedicated protectors. Emperor Penguins are a prime example (even though they are only serially monogamous). There is no way that the offspring would survive without the two parents working together. This extra dedication to the child helps ensure that the parents genes get passed on. It could also be argued that monogamy increases the likelihood that an organism will bear multiple children.
The feeling of love is evolutionarily beneficial. Forming a strong bond with a mate encourages protection of both organisms, and of their children.
You might think that this is a grim way to view love, but I find it fascinating and beautiful. This line of thinking has greatly improved my opinion of human nature.