My guess is that he wanted a reward for his role in the defense of the sacking of king's landing. It was the first time he did something noble not only for the city, but for his family. At that point, he probably felt worthy enough to claim Casterly Rock.

My guess is that he wanted a reward for his role in the defense of King's Landing. It was the first time he did something noble not only for the city, but for his family. At that point, he probably felt worthy enough to claim Casterly Rock.