part of The Joy of Pair Bonding

Because it's fun, obviously.

Seriously, there are a number of reasons why an adult would choose to pair bond. Here are some of the more commonly cited reasons:

Childrearing and The Future of the Species
Since human reproduction is sexual (ie we don't bud), it takes two to tango. A male and a female are required to produce offspring, outside of a laboratory setting. Since infants are weak, helpless, etc., they need at least one adult to provide food, shelter, a role model, etc. Bringing up a child is a massive undertaking; it's a lot easier when you have someone to share the work with - especially if that someone has a 50 percent interest in the offspring's genotype.
Genetics and Social Instinct
Humans are social animals. Our brains are hardwired for it, and our peers train us for it. Pair bonding is a specialized form of socialization between two individuals. Your genes are telling you to do it; and so are your parents.
Fear of Dying Alone
Loneliness is rampant in today's society. Many people choose to pair bond for the companionship. However, increasing rates of late-life divorce make this less fo a sure thing.
Love
You may experience increased feelings of happiness, well-being, and giddiness around your partner. This is good.

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