Web Forums. Why do so many people act out on them?

Its amazing how aggressive people are on forums, they act like the kids that bullied you at school (unless you were the bully of course),  and tell you helpful things when you disagree along the lines of “not living in the real world” . They often base their idea of reality purely on the own personal experiences and cannot conceive that anyone will have other obstacles to overcome that they have not. Ergo everyone else is stupid or lazy or deceitful and anyone who’s life has taken a different path is simply to be discounted.