[bitmasher9]

I think one major aspect of friendship is the random reward (similar to a casino). In any given social interaction the outcome could be extremely negative to extremely positive. While a good friend will heavily skew positive, there is still a range of outcomes.

Humans can have this range of social outcomes naturally because all parties are constantly in different moods. Sometimes humans careful choose their social behavior to manipulate others, and this is generally frowned upon. A machine cannot have a wide range of social rewards without being manipulative.