What causes us to feel sad when "alone"? And why?

Some philosophy here.

Why do we feel sad when alone (sometimes)? Why are we driven to find a partner in life to be with even after we have fertilised her eggs? (or whatever the female equivalent is).

I understand we have a ‘drive’ in order to seek a partner, but I am just wondering what the cause for this is. I am unsatisfied with the single evolutionary explanation that it is to have children and bring them up. I want to know about the cognitive factors working here and possibly how they developed.

Now gogogo speculate and philosophise.