Fonte Reddit :
People are going to say that he took the black so his son could rule because that has sprouted up on the wiki and been taken for fact despite that there is nothing to even indicate that in the books.
I married young, to a bride of my father’s choosing, a Glover of Deepwood Motte. Ten years we were wed, or near enough as makes no matter. She was a plain-faced woman, but not unkind. I suppose I came to love her after a fashion, though our relations were dutiful rather than passionate. Three times she miscarried while trying to give me an heir. The last time she never recovered. She died not long after.”
Dany put her hand on his and gave his fingers a squeeze. “I am sorry for you, truly.”
Ser Jorah nodded. “By then my father had taken the black, so I was Lord of Bear Island in my own right. I had no lack of marriage offers, but before I could reach a decision Lord Balon Greyjoy rose in rebellion against the Usurper, and Ned Stark called his banners to help his friend Robert.
Este é um excerto de "Daenerys I", capítulo 12 em A Clash of Kings .
Esse é o mais próximo que temos de uma explicação para ele tirar o preto, o que, como você pode ver, não é uma explicação.