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[ASOIAF/GOT] Why was Jeor Mormont in the Nights Watch?

Did he decide to join because he was ashamed of his son's crimes? Or did he join because he thought they needed men? Or did he break some laws himself? 

And what about the other noblemen in the Nights Watch like Denys Mallister?
gpt
Jeor Mormont joined the Night's Watch before Jorah committed his crimes. He abdicated several years before Robert's Rebellion. 

Not all of Westeros' houses view joining the Night's Watch as a form of punishment. Some men from certain houses, mainly Northern houses or houses with strong First Men links, will willingly join the Night's Watch. Obviously Benjen Stark, Jon Snow, Waymar Royce and Jeor Mormont are good examples of that. Denys Mallister probably also joined by choice. There's a number of other men in the Night's Watch who are of noble birth who we're unaware as to the reason they joined (Mallador Locke, Ottyn Wythers, Wynton Stout as examples). 

The ones we know that were forced to join are Alliser Thorne and Jaremy Rykker (wrong side of Robert's Rebellion). Samwell Tarly was also forced to join, not because of crimes, but because his father's a dick.