Martin:

Because it is never absolutely true, and if you pretend you're just being honest, you're just being a sadist. A balance of encouragement and helpful criticism will help another person a lot more than blunt invalidation even if it seems justified because blunt invalidation loves no room for hope.

On the other hand, if what you are trying to do is not help but crush, then it is of course the method of choice. Depends on what effect you are trying to create, I guess.

A