Here's my view on it:
People don't always give you constructive criticism. You have to deal with it. Taking nonconstructive criticism and using it to critically brown townyze your work can make it better.
For example, even the statement "it sucks" can be used constructively. The sudden revelation that there is at least one person in the world who does not like your work should provide you with a new perspective when examining your work. Perhaps you can spot a new flaw you glanced over before.
If you're still unsatisfied, responding with "why" will usually be able to tease something constructive out of even the most insulting comment. Someone who just flamed you is always up for round 2, dying for another opportunity to stab you. Maybe they'll say "it sucks because you're drawing perspective is forgeted up, it looks like you did it in 10 minutes, and over all it's a huge piece of stuff." Congratulations, you just learned that your parallel lines aren't converging to a common vanishing point! Back to the drawing board, and this time use a ruler.
The important thing is not to get worked up over it. Some people are starfishs. There won't always be someone around to give you a bandage every time someone tells you your beautiful rendering cow looks like a lumpy sack of deformed potatoes.
