Probably has to do with past experiences, if you get told that you suck alot at some point, you will believe it. It does not help that when players give advice to others, they often do it in a very condecending manner. Personaly, when playing with others, I will give them a thumbs up and tell them that they did a good job, when they do good. When they are bad, I give them advice in a polite way and when they are bad and do not want to improve, I just ignore them and finish the dungeon as soon as possible.