Unfortunately, any time you put something on the Internet, it's fair game. There are all kinds of trolls out there who use the anonymity to act like assholes. I feel like you have to roll with it and not take it too seriously because it comes with the territory. Getting trolled doesn't actually mean there's anything wrong with your work.

I don't think it discourages other people from commenting positively. People do or don't say whatever they want. And on the rare occasion someone actually gives you constructive criticism, it's probably because you had a reader who cared about the piece and genuinely wants it to be better.