Then comes the double edged dagger that is character critique. They are ones creations, ones children, and some stranger wants to come along and change these beings one has crafted from nothing? And even if the author keeps her ego in check there is the danger that an outsider might not understand the unique traits that make the character what it is and any suggested changes might ruin the final product, or at least overwrite the author's voice.
And heaven help critiques of jokes and humor...
Still, with all the pitfalls and traps it is critical to get that second opinion, do remember who you are and get someone you trust, but find yourself a critic.