The reason for breaking a piece of earthenware under the chuppa is to remind the groom to not get caught up in his desires, and end up going to Gehinnom (Hell). A groom must purify himself in the marriage relationship. Breaking the earthenware vessel is also a hint to him that if his wife turns out to be bad, G-d forbid, he shouldn’t mistreat her or divorce her, but know that because of the evil way she acts toward him, he will never see the entrance to Gehinnom! One should see to it that he never comes to the point of divorce.

–Advice: Marriage #7