Perceptions and understandings never have to be perfect, they are always only our current working hypotheses, subject to change whenever we see actual cause to modify them. Actual cause, not doubt. Doubt is only relevant if we are assuming perfection in our perceptions and understandings. Since conscientious people never have such an assumption, doubt is never relevant.
Doubt (self-doubt) only has value as a possible restraint upon arrogance. But for those without arrogance, doubt is evil, because it obstructs their acting upon their perceptions and understandings. (Their working hypothesis perceptions and understandings; if they do not proceed with confidence in their working hypotheses, they will never encounter more real data with which to better modify those working hypotheses, which might be 90% accurate already.)
For instance, say bright children encounter the concept of "regret" for the first time. They think about it, looking for what possible value it might have for their life. After a few minutes of hard thinking, they see zero benefit. So, having no self doubt, regardless of what value others found in that concept, they toss it out of their lives forever, being endlessly benefited by its absence. Of course should someone else show them a positive benefit to regret, they would reconsider it. Absent that occuring, if and whenever, they enjoy a much happier life.