I really am not a night watch arc fan of the books. I think it is criminal for a group of teenagers and pre teens to force a man to celibacy to save the realm for the rest of the world to find love. Duty is not true duty unless you know what you are protecting, you can not make a promise to sacrifice yourself completely and utterly, to become a void until you have lived and loved and understand how important those things are. The show got this correctly by showing Sam's progression, his true promise was not his night watch vows but instead his promise to do anything to protect Gilly, even if he has to make a second promise to stay alive and come back to Gilly for Gilly couldn't be Gilly if she didn't have her Sam in her life, that he was protecting not just her well being but also her happiness.

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Jon made a different decision than his father when he chose to return to Castle Black instead of staying inside the cave. He could have love or he could have duty but he could not have both. The only way he could understand the weight and thus the importance of duty was for him to love Ygritte.