Offer a guess, and describe what you think drives Goodman Brown. What is he seeking? Does he find it? Did he already have it only to lose it?

It's reasonable to think that Goodman Brown is seeking to test the strength of his Puritan faith. He seems to think it is unshakable, and so he actively seeks to meet with the devil to see if he can withstand any temptations he might have to offer. He seeks confirmation that he is on the right path with regard to both his earthly life and his afterlife.
Goodman Brown's faith remains intact, and one might argue that since the Puritan faith was rather dark in its outlook on humanity, what happens to Brown in the woods confirms his religious inclinations. Puritans tended to search themselves for signs of their own sinfulness, and since Brown spends an evening consorting with the devil, he acknowledges and embraces his own imperfection. Moreover, Puritans tended to be judgmental of others, and Brown becomes, in the words of the narrator, a "stern, a sad, a darkly meditative, a distrustful, if not a desperate man," after the sojourn in the woods.