You should already be your own biggest detractor, all day, erry day. If you aren't, you're in for a rude awakening.

There's no bravery, or merely even resilience, involved with listening to and learning from criticism of one's product.

Sometimes ideas are just plain bad. Sometimes implementation are just plain bad. If somebody wishes to express such sentiment, then that's a perfectly fine thing to do, and in fact it should be encouraged.