Perfectionism is self-destructive simply because there is no such thing as perfect. Perfection is an unattainable goal. Additionally, perfectionism is more about perception - we want to be perceived as perfect. Again, this is unattainable - there is no way to control perception, regardless of how much time and energy we spend trying.
During relaxation we drop our guard. Particularly in conversation. Relaxed conversation leads to openness. And in openness we often reveal what should never be revealed.