We get it at an early age, from teachers and parents and peers alike. In high school we dread it and by college we nearly fear it. For those of us who continue to write after graduation, we must learn to accept it as part of our career. It comes at us from all directions at all times, and even from within. It is - criticism.
There are some writers who cannot take criticism. If someone doesn't like their work, well, then it must be their problem. If an editor claims their dialogue is too weak, the editor needs a better imagination. If a friend doesn't understand the plot line, that friend must have an IQ of a doorknob. Their own work is invincible and stands above that of everyone else, and no one can tell them otherwise.
Unfortunately, TOO many writers are like this. Most of them will never be published because of it. That is why I say writers, not authors. Their ego and unwillingness to revise keeps them from improving and being understood. Authors know they are never above criticism or revision.
Criticism can be painful, especially when it's coming from someone who doesn't like your work. People can be harsh, spiteful, and downright rude. Most people don't know how to give constructive criticism - that is, they don't know how to say "this is awful" in a way that helps the writer improve. All they know how to do is state their opinion, which frankly, doesn't provide a lot of solid information about what the true problem is.
The next time someone criticizes your work, seek to understand what specifically about it they don't like. Perhaps your grammar needs a bit of polishing or you're simply missing a bit of vital information. Maybe your syntax needs work, or your run-on sentences are preventing the reader from understanding your point. The worst that can happen, since there are no strict rules for solving these problems, is your plot or characters are hurting your story. Until you can get past "this is dumb," and discover the specific problem, you can't fix it.
To become a great writer, you need to give in to the fact that your work will be criticized - there's nothing you can do about that. What you can do is be open to receiving it, learn not to take it personally, and use it in a positive way. Learn to ask the right questions to get the answers you need, and thank your readers for helping you. When the going gets really tough, remember that massive success (post-revision, of course) is the best revenge on your critics.
