How is young Goodman Brown stupid?

I don't know that I would describe young Goodman Brown as "stupid"; he is, perhaps, misguided and selfish, but I wouldn't call him stupid.  He knows that he shouldn't go into the woods.  He knows that he shouldn't leave his wife, Faith, who claims to be troubled with distressing dreams when she is alone.  Brown even feels guilty for leaving her behind, and he advises her go to bed early and say her prayers.  She hopes, for his sake, that he "'find[s] all well, when [he] come[s] back.'" Such a statement seems to foreshadow that he will not. 

Brown claims that "'after this one night, [he'll] cling to [Faith's] skirts and follow her to Heaven.'"  Faith symbolizes his own faith in God, faith that he purposely abandons at home, just as he leaves her behind.  He plans to commit some sin in the woods, and the narrator describes his "present evil purpose": it seems that Brown goes into the woods, feeling sure that his faith will be there waiting for him when he returns.  However, this is not how faith works; we cannot choose to pick it up when we want it and walk away from it when we don't.  I don't think Brown is malicious, but he is selfish; he wants to gratify whatever sinful desires he has without having it harm his faith, and so he -- literally and figuratively -- leaves his faith behind without realizing that he will not "find all well" when he gets home.