Mara Wilson, nostalgia critic, and commentors

First of all, let me remind you that English is not my mothers tongue, and I suck at punctuation.
This is from Mara Wilson's blog marawilsonwritesstuff.com:
"...someone told me they had found my Facebook page through the Nostalgia Critic. They pointed out, once again, that he did not care for any of the movies I’d made as a child. [...] At first I ignored it, but after a rough week and one too many “Hey, Nostalgia Critic hates you!” posts" (yeah, I know her quotation marks are supposed to be replaced by something else, but I just suck, OK?)

Why are some people so mean to people they don´t know (yeah, saying things along the lines of “Hey, Nostalgia Critic hates you!” can be considered mean)? Why? I've read a lot of Mara Wilson's tweets, and I realize that I don't agree with her on politics, but even I wouldn't be mean like this, so why do people say al ot mean things about her? Based on what I've read on her blog she seems to be a nice girl (I hate using the word "woman" to describe someone who's about my age, because that makes me feel old), so why are people so mean? Just leave people alone! Life is to short for making other people miserable! We should be making people happy!

For the sake of Darwin, Carl Sagan, and Neil Degrasse Tyson, just shut up.