On the drive to school today I had a really good conversation with my two kids about gamemastering (which is a sign I'm raising them right). The question was, if a player is having a legitimate problem with another player at the gaming table, should the GM intervene? My kids had two very different responses.
One said yes, because as the GM you have made a social contract with the players that they will have a good experience and be treated with respect. If one player is giving the other a hard time, it's your job to say something to the offending player about how that isn't acceptable behavior.
The other one said no, because that's triangulation, which isn't healthy, mature social behavior. If the offended player comes to you complaining about the behavior of another player, you should tell them to take it up with the player one-on-one.
I had my own answer, but what do you think?