Most of the time when someone criticizes your work they are not discussing art or design or music or film etc but rather expressing their own personal feelings which are mostly based on insecurities. Any amount of civil discourse will eventually if not immediately show this. They cry out not because they see a flaw in your work; if this was the case they would spend every minute of every day on the web "helping" people who have not produced a perfect piece, no, but rather they do so because they suffer a short coming in their own being that they find a need to express it so they can be heard and the sand from their fannies rinsed out.
Its not as if they are going to get the artist or band to start writing or drawing solely for them, so you have to wonder what the point is. "Hey John, Paul, Ringo, Steve from Manchester says he finds this new song boring....lets change it immediately and make sure we write songs that he likes". Its massively egotistical if you think about it, a very self absorbed attitude to have. If it were the real world you would have to get up very early in the morning to make sure that every shop you passed could benefit from the knowledge that you don't like their product and you wont shop there. Its bizarre behavior. even on the web.
Saying nice things to people does a lot, it supports them, it helps them, it motivates them and others, it helps health and mind...Saying bad things makes you look like a dick. Simply don't do it. I promise you, it doesn't help. If you dont believe me go find a music video you really really love and find a comment you disagree with. What do you think of that person? See. Hurts huh.
If you really cant help someone what is the point in saying something? If you can help someone- this is a great way to say it:
Also- Stephen Fry on "trolls":