Why are people so quick to jump to conclusions?

This question is not directed to the outside world it is directed to the fluther world only. Recently a new user in need of a hand got accused of being a troll. Seemingly only for the fact that he/she had similar letters in their user name as a past troll. His/her question got modded because a few people thought he/she was a troll. I know this person isn’t a troll because I’m pm-ing him/her right now. So why are people here so quick to jump to conclusions?