Humans are gregarious creatures as opposed to say Bengal tigers. We create societies and culture. From a biological standpoint, it would seem that gregariousness - the desire to interact with other humans is hard wired. However, as to what forms this interaction takes, that is probably a combination of the individual's underlying temperament and upbringing.
One aspect I have noticed is that some people have a deep-seated fear of being different, of deviating from the expectations of the society at large. It occurs to me that this wanting to stay with the herd mentality might be tied to a survival instinct. As a general rule when under threat, it works well. However, perhaps it is employed in some people even when there is no threat, or the threat simply consists of being noted as different.
The desire not to be alone may hail from the same place.
Then there are the introverts who need time alone to recharge, and the extroverts who need to be around people to recharge. If a person is an extrovert is it really such a stretch to imagine that s/he does not particularly like being alone? Being alone does not fulfill an extrovert's needs. An extrovert might be more driven in general (psychological factors like esteem issues aside) than an introvert to have a partner.
You asked a big question! These are my initial thoughts.