Why did Benjen Stark join the Night's Watch?

[The Fat Huntsmen]

Although it seems problamatic that Benjen joined the NW when Ned only had one child as heir, you can look at it the other way and think that maybe Benjen swore the oaths for the specific purpose of removing himself from the succession.

Ned was well-loved after ruling the North for 15 years but when he first came home it was with a southern wife and after being raised in the south. Perhaps there was a faction of the northern nobility (including Lady Dusten no doubt) that would have preferred northern-raised Benjen - and so he took himself away to avoid being used against his brother.

This makes the most sense to me.

[Lion of Judah]

I'll probably screw this up, but the short version of the most popular theory is is something like...

Benjen and Lyanna were very close, and he helped her find armor to wear (as the Knight of the Laughing Tree) for the joust at the tournament at Harrenhall during the year of the False Spring. Rhaeger was infatuated with her after her success, named her the Queen of Love and Beauty, they ran off together, a war started and she died. Benjen felt guilty about helping cause all this, and took the black.

Or maybe he was just a third son who wanted to look for glory at the wall! :dunno:

I love this theory, I had a discussion a while ago about this very topic. It is still debated wether the knight was Lyanna, but this fits her character like a glove. I believe that she was indeed the one to defend Howland's honor the description of the mystery knight fits a woman to be frank. Small in stature, armor didn't match as though it was just thrown together, none of it fit so it didn't belong to the person wearing it, the weirwood tree on the shield. A man could make do with another mans armor at least how to make the ensemble look presentable. However the knight of the laughing tree didn't look very presentable and so out of place that Aerys felt threatened.

[Faceless Man Friday]

Why did Benjen join the Night's Watch? The answer may be in Winds of Winter. Remember "Winter is Coming" and the Night's Watch may need a Stark when the Others come. After all one of the biggest unanwered questions in the whole series is what has happened to Benjen.