Why are emotions considered 'bad' online?

I’m on several websites/blogs/boards etc, and it seems that whenever someone is upset, without fail, someone else will come along and tell them to shut up/grow up/get over it and the like. I’ve seen happy people told to go ‘f’ themselves, suicidal people told to ‘go ahead and do it’.

Why are online displays of emotion considered uncool or stupid? I could understand this if it was only on immature boards that I’ve seen this behavior- but I’ve seen it on communities that would normally consider themselves open and safe.

What do you think causes this mentality online?
I’m not just talking about calling people names, I really mean the whole idea that anytime someone shows real emotion it is seen as uncool or overreacting, even with evidence to the contrary.