Laws dictate a prophet must never get married, and live a plain,simple and cloistered life. MC is very disappointed and sad at this, since she desires to find true love, live independently as she wishes, and wear nicer dresses. 
This is why in her current incarnation as a duke's daughter, she wants to live the free, pampered life she never could get as a prophet, as well as find a good partner this time. This is also why she doesn't want to be robbed of her 2nd chance at freedom by becoming a  prophet again.