How should one handle criticism?


If the criticism is superficial, then let it be. It is one of the Navarasas (the nine emotions, which are, love, laughter, compassion, anger, courage, terror, disgust, surprise, and peace).
If you want something bad to happen to another from the core of your heart, from the depth of your being, then you are harming only yourself. The person being criticized won’t be affected. But that doesn’t mean you should be superficially positive, and never point out mistakes.
If an apple is bad, saying the apple is good is insincere. If someone asks you, ‘How was the food’, and you say it was very good, but you couldn’t eat it then that is not honesty. Where there is a problem, it must be pointed out, but without any hatred in your heart, otherwise the feeling will harm only you.
If somebody else does it, then let it be. What can you do? It is their rajoguna and tamoguna which makes such negative tendencies arise in them. But if you become negative towards them because of that, they you will also be in the same basket as them.
The three gunas, rajoguna, tamoguna and sattoguna are an integral part of nature. You just be a witness to their play and display. And if you want to explain it to somebody then do it because you care for them, do it with compassion. Tell them that "You have spent two hours complaining about somebody today, this will severely affect your creative or positive energy. The more you complain the more energy you will lose."
Tell them to complain about somebody for an hour and observe how much energy they have lost. When people start complaining about others they forget the time because they find some form of enjoyment in it. Like a patient enjoys talking about his ailments. If he has ten visitors, he’ll tell all ten people the same thing, ‘All morning I have been having leg pain. I could not sleep all night because of the pain’.
All the crying and drama is repeated over and over again. He should do something else instead of harping on and on about it. Write a poem maybe, or do some other creative thing. Then he can be happy and also forget his ailment.
Anything that is repeated, gets deeper into the consciousness. That is why mantras are repeated again and again. So don’t let complaints get deeper into yourself, instead do some japa (meditative repetition of a mantra or name of a divine power).