Love can be an irrational thing. It makes people go to great lengths, but not without any difficulties. Sometimes pursuing and being with the person you love isn't easy, especially when the people around you are strongly against your relationship. For royalty, this struggle is unavoidable.
Historically, kings (and queens) dictated who their children could and could not marry. As heirs to the throne, princes were only allowed to marry women of high status and good reputation. Whenever they fall in love with someone who doesn't meet their parents' requirements, the princes' parents do everything in their power to keep the prince from marrying who he loves. This is what happened to Peter I of Portugal.
The son of King Afonso IV of Portugal, Peter's love for Inês Piras de Castro was forbidden by his father because Inês was illegitimate. Despite this, Peter continued his relationship with Inês, even going as far as claiming that they were married. Afonso, in an effort to end the relationship, had Inês sent to a monastery and later had her killed by three of his henchmen.
However, Peter still loved Inês. After his father died, Peter sought revenge on his wife's murders and successfully killed two of them. As the new king, Peter had Inês body dug up, dressed up in the queen's clothing, and had her crowned, making all his citizens pay homage to a corpse.
Some relationships are so strong that the last beyond the grave. For Peter, despite his wife having been buried underground for a few years, he still brought her back to his side and pledged is loyalty to her. Love truly knows no bounds, as some people are willing to continue loving a corpse as though it was still alive.