I believe that she's had many lovers and spouses over the course of her long life, and not all of them were princes. Some peasants, some soldiers, some even outcasts who got her attention by doing some good deed or another. All of them, however, eventually were claimed by the reaper. Murder, burial in an avalanche, disease, old age, accidental structural failure- they died in numerous ways. And though Celestia tried her best to move on and live in the moment, eventually all those lost loves took their toll, and the goddess-princess found herself living in fear of falling in, and losing love once again.