Why do hockey players fight?

I saw my first hockey game on Saturday (the San Jose Sharks versus the St. Louis Blues) and was appalled at the way the players can get into fights. Their protective gear is thrown aside and they go at each other with their bare fists until one finally emerges as dominant. The referees do absolutely nothing to stop them, and the fans around us were on their feet, cheering them on in a disgusting, bloodthirsty way. To me, the fighting seemed unsportsmanlike, overly violent, and completely unnecessary. It ruined the game for me. Why do they fight, and why does no one stop them? In (American) football, players are penalized for “unnecessary roughness”. Why not in hockey?