There is a major difference between authentic encouragement and insincere flattery.

Insincere flattery doesn't have the same power to impact another person. Insincere flattery might be made up of positive words. But the person saying them doesn't have the necessary feelings, even if he does have good intentions. The words are even less powerful if the intentions are purely manipulative.

Authentic encouragement is when you sincerely believe in someone's abilities. The words of authentic encouragement flow from your heart. They have the ability to penetrate someone else's heart and mind. The person knows that you actually believe in him. This is conducive for him believing in himself.