It is easy to see what you have had.

"I need a friend, I need a love, I need someone who'd miss me.
When I get home I don't want to be all alone."
- "I need a House" by Marie Serneholt

No matter what we do, we generally don't want to end up in solitude. When something really outstanding comes our way, we wish to share our overwhelming exuberance with a companion. When troublesome clouds give a sinister gloom to our horizon, we feel the need for someone's support, even if it is merely moral.

Every once in a while a person comes along in our lives that cheers us up, who we feel we could be together with. If we are lucky, we feel right, and if we are very lucky, we stay together with that person. It is the age-old issue of finding one's other half. Some appear to have more than one other 'half' and some go through their options on a regular basis. In some cultures it is not only acceptable, but encouraged for some men to have many other 'halves', amounting up to several complete people. In others, a person can only choose to bind oneself to a single other 'half'. So why do we do it?

The obvious answer would be the driving force of all living - reproduction.

The less obvious answer is emotional value. In times of great risk, some people believe that it is better to risk when you have nothing to lose. While that may be a safety net of sorts, it has a major negative influence on motivation. Sometimes knowing that you have something or someone to lose is all that you need to get you going. In essence, it is really fear that helps you, and fear is a powerful motivator for getting things done. Moreover, having someone around creates a sense of security, which is invaluable in the cases where that someone is not at risk. It is a safety net for the times you have something else to lose.
It is fairly obvious that while people can be competent enough to fare well alone, it is not the preferred state of affairs. Good company is invaluable.