The human baby requires a long term commitment to get to adulthood, and, it requires more than one person to do it. Unlike other animals, such as Cheetahs,whose cubs are suddenly abandoned after they become self sufficient, the human baby needs years of support. The best way to do that is for a strong commitment by two people. That's what marriage is for; it makes it a legal as well as a moral contract. However, after the child rearing,the only reason to stay together, other than love for each other,is the kids. Thats a social thing. I believe it was Margaret Meade, the anthropologist, who said "To expect two young people to swear everlasting love and togetherness is silly."(I paraphrase) Back when people only lived 30 or 40 years, that may have worked, but, not now. If you have that, you are one lucky person. It is rare today, and, a divorce rate of 50% proves it.