Doubt is a feeling of uncertainty about the truth, reality or nature of something. As such it is a bit uncomfortable. But doubt is extremely valuable to us. It protects us from being deceived and when we are deceived we are at risk. We are at risk for behaving contrary to our own best interests or contrary to how we would want to behave if we knew the true state of affairs. To learn to doubt is to learn a form of self-defense.
As has often been pointed out, we are often guilty of self- deception. It seems that we often want to believe something and that in our desire to believe, we actively seek confirming evidence and avoid conflicting evidence. In this state we shun doubt. But, whatever your goals, the best foundation for achieving them is the truth and so it is necessary to be on guard and to repeatedly ask yourself if you want to know the truth or if you want to believe some comfortable deceit.