England
Piers Gaveston
Kings always have favorites. And many kings had lovers. It is generally a mistake, though, for a king to promote his lovers to positions of power to the detriment of the established elite, but this is what Edward did.
Most of the time, a king's lovers are women. Favor shown toward them might mean lavish gifts or a presence at court. But Edward was homosexual, so the favors he showed were political, making it a far more dangerous game.
As a case in point, Edward's friend at the time he became king was Piers Gaveston, a commoner of modest wealth. At the coronation, Edward showed more honor to Gaveston that to the queen (yes, he was married).
Even this might have been manageable, except that Edward deeply mistrusted his barons and so instinctively did not turn to them for advice. Instead, he neglected them and relied on Gaveston and other members of the royal household. If, like Gaveston, they came from non-noble background, Edward granted them lands and titles so they could become part of the court.
This double tendency, of pushing away the traditional councillors of the king while at the same time favoring outsiders, served to make Edward politically isolated. If all went well in the world, Edward could get by. He might not be popular, but the prestige of the royal office was by this time strong enough that few would dare to rebel openly.