(I'm really sad that I wasn't able to get this in while House Mormont was still House of the Week.)

It's a common question about why Jeor Mormont abdicated his position as Lord of Bear Island and joined the Night's Watch.

The most accepted reasons don't seem to paint the whole picture for me, and in fact highlight the odd circumstances surrounding his departure.

[Jeor] explains that he didn't want his son to be waiting around for him to die.

...

Jeor wanted Jorah to have a chance to rule before he was an old man.

Um...who cares? There are plenty of old lords in the North. And Jeor seems to have ruled Bear Island for quite some time from quite a young age. He was only 58 when he was voted Lord Commander of the Night's Watch, so he definitely ruled Bear Island in his "prime" and would have no limited empathy for an of-age heir apparent. Especially when he had no "spare".

In the north it was common to join the nights watch once your son grew to be a man.

Key word: WAS. This custom has fallen out of practice. Younger sons of lords still join...sometimes. But in general the Wall is a place to put reavers and rapers instead of wasting them with execution.

Bear Island is quite close to the Wall and as a Northerner he probably wanted to do his part in helping the Norths defense.

He could have done that from Bear Island. It's in a strategic position to defend the Northern capital (Winterfell) from sea attacks by the Wildlings.

Someone suggested that Jeor's wife dying caused him so much grief that he left, which is the most plausible explanation.

Ehhhhhhh, not quite. That left him the most plausible excuse, but that's not the explanation.

I propose that Jeor Mormont is was an aging gay man. My main source is something that Craster says to him in season 2 episode 1:

Oh, aye! You chose the path with nothing but BOYS on it.

Jeor doesn't deny, or say anything about the Night's Watch. In fact, it stumbles him and he can't think of a new question and Craster is able to shut him up and send them off to bed.

Jeor being a closeted gay man would explain a lot.

1. He joined very soon after the death of his wife. Jeor Mormont is a Northman through and through...and Northmen are loyal. He never would've left a behind a wife, but as the only male son, he never would've abdicated his spot without making sure there was someone else to take his place.
2. Jorah was a man when he left, but he left just ONE man. Usually lords will aim for an heir and a spare. Jeor had no spare, which is why Bear Island passed to his sister and nieces.
3. He only had one son. Everyone else that is a single child family gets a long explanation of the number of miscarriages and amount of grief associated with trying for a baby. No such explanation for Jeor. He did, however, set up the chances for a spare by arranging a marriage for Jorah. (Who, by the way DOES get an explanation for how his first marriage was plagued with infertility).
4. His pet raven. Pre-1970s, gay men were "confirmed bachelors" and they often were very close to their pets. It's still a common stereotype that single gay men fucking love their pets.
5. He joined the Night's Watch. All-male institutions are known for dude-on-dude experimentation. For example, the sex act of "frotting", that is rubbing two penises together, is also known as the "Ivy League rub" from when the Ivies were all-male.

So, in conclusion:

The Night's Watch vows his newfound widowerhood an adult heir = the perfect time to leave and enjoy his life in what is essentially the only gay man's haven in Westeros. (Aside from Dorne, but that's no place for a Northman.)

Once there, he made the best of his life, as he was finally happy. He quickly rose to a position of power and implemented new and effective strategies.

(This obviously invites many tinfoil references to jilted ex-lovers of Mormont's. My favorite is Aliser Thorne.)

TL;DR Jeor Mormont is gay.