What specific tenets of puritan beliefs changed young goodman brown to a gloomy end?
The Puritan doctrine requires extreme faith and commitment on the part of its congration. Part of the issue for Young Goodman Brown is that he believes that all members of the congretation are godly and pure and that he is the only one who dares to entertain thoughts of entering the forest. He wants to simply overcome his fear, and perhaps internal, wicked curiousity, enter the forest, and then return placidly to his wife, Faith. However, Faith is his undoing.
Brown notices that many upstanding members of his church are in the forest too, and his companion hints that his ancestors have also made this journey. This causes him some doubt and anxiety, but it is mostly the fear of being seen by these "pious" leaders rather than his wondering why they are in the forest in the first place. However, when he sees his wife in the woods, he loses his faith, both literally and figuratively, and storms the ceremony to witness her loss of innocence, and his own.
Though whether the incident was a dream or not, Brown is still ultimately disillusioned with his religion, his peers and his wife, and he lives a solemn, gloomy life.