~Companionship

Humans are social creatures. We evolved/where created (take your pick) to be surrounded by others. This is why people on deserted islands go crazy, and why prisoners sometimes get attatched to inanimate objects, and why there is a Companion Cube in Portal. This is also why Frankenstein's monster became a heartless murderer.
  One reason we love company so much is because we can communicate our feelings with them. Sure, you can talk to your pencil/computer/companion cube all day long, but it will never actually respond, which takes the fun and instinctual satisfaction out of it. (although if it does, you might want to satisfy your need to communicate with one of those people dressed in long white coats...). Talking to others usually helps us better understand our emotions and act accordingly. When we are deprived of this, our emotions become a jumbled, nonsensical mess that is difficult and sometimes painful to sort through.
   One more reason why we need fellow people is love, because we don't just have a natural urge to find a mate and reproduce; human relationships are more than that. It's difficult to state exactly why we need this so badly, but I think we can all agree that a person who goes their entire life without ever experiencing love wouldn't be very happy, yes?

"The symptoms most commonly produced by Enrichment Center testing are superstition, percieving inanimate objects as alive, and hallucinations. The Enrichment Center reminds you that the Weighted Companion Cube will never threaten to stab you and, in fact, cannot speak."