Responsibility weighs down on people.
At this point, Jon Snow has been elected as Lord Commander very surprisingly. He definitely was not ready or expecting to have to shoulder such a heavy burden.
In his unreadiness, he took to Maester Aemon's advice:
“Allow me to give my lord one last piece of counsel,” the old man had said, “the same counsel that I once gave my brother when we parted for the last time. He was three-and-thirty when the Great Council chose him to mount the Iron Throne. A man grown with sons of his own, yet in some ways still a boy. Egg had an innocence to him, a sweetness we all loved. Kill the boy within you, I told him the day I took ship for the Wall. It takes a man to rule. An Aegon, not an Egg. Kill the boy and let the man be born.” The old man felt Jon’s face. “You are half the age that Egg was, and your own burden is a crueller one, I fear. You will have little joy of your command, but I think you have the strength in you to do the things that must be done. Kill the boy, Jon Snow. Winter is almost upon us. Kill the boy and let the man be born.”
-A Song of Ice and Fire: Book Five - A Dance With Dragons, Chapter Seven (Jon II).
This rings in Jon's mind repeatedly all throughout the last book (A Dance With Dragons). He takes it:
- He often thinks that he must make the "hard" decisions
- He alienates his friends thinking that they are distracting him and that all they seem to want to do is play
- He sends Sam away to study at The Citadel
- He starts to see and think of himself as a man, not a boy
Jon makes some hard decisions, some which many men take no liking in (some even voice their displeasure); however, Jon was extremely pre-occupied. He had to deal with:
- Stannis encamping at and demanding the Night's Watch's support in his crusade
- Being offered to be legitimised and made Lord of Winterfell, but having to reject because of honour and vows and stuff
- King's Men and Queen's Men voicing their distaste and their disagreements
- Sending away his best friend Sam as well as Maester Aemon, along with Dalla's babe
- His recent loss of the love of his life, Ygritte
- Melisandre's weird obsession with his "blood" and "power"
- Winter fast approaching and way too scarce resources for everyone
- Free Folk being given a part of The Gift to settle
- Free Folk residing in Castle Black
- Minor fights between the Free Folk and the Westerosi.
- Lack of money and subsequently putting the Night's Watch into debt with the Iron Bank of Braavos
- Janos Slynt's insubordination and execution
- His cousin, Alys Karstark, taking refuge in Castle Black and her subsequent marriage to the Magnar of the Thenns
- Finding out his sister, Arya1 has been forcibly detained in Winterfell and married off to the newly legitimised Ramsay Bolton.
- Threat of the Others looming
- Pink Letter
Yes; he knew full well that he had angered a lot of people and had made some tough decisions, but he fails to see a secret assassination plan since, well, it was a secret and he had way too much going on at the time.
1. Yes, we know she's fake, but he doesn't.