Friend

If I had all the money in the world but know one to enjoy it with, I would be depressed. Admittedly that's an extreme example, but the truth is that all of the enjoyable things in the world get old quickly without a friend to share them with. On the other hand, if I was experiencing all the hardships in the world, I would go insane without the aid of a friend. People that don't have anyone to socially interact may lose their minds and develop an imaginary friend just to ease the loneliness. Human interaction is necessary.

It is simply the way that we are wired.