Because the world is full of psychos and mentally unstable people who disguise in the form of sane humans. In most cases you will be criticized or humiliated because of psychological issues that the attacker suffers from and not because you are really bad or defective.
The majority of people don't have a high self esteem.
Brave ones who actually have the courage to follow their dreams are a minority.
Most people feel insecure and hate to see others doing better than them.
Do you still think you should feel bad when you get mocked?