What exactly happened to Young Goodman Brown that made him live a gloomy life?

At the beginning of the story, Brown appears confident in his ability to choose between good and evil, but once he stands before the Devil's altar, he can no longer believe that good always prevails. He becomes a profoundly disillusioned man, who sees wickedness everywhere, even in those closest to him. Some critics have interpreted Brown's resulting distrust and isolation as the result of a guilty conscience; he cannot forgive himself or others for hidden sinfulness. In the end, Brown is unable to accept the duality of human nature-that a person can possess both good and evil qualities—and for this he suffers.