Why the ending of Korra makes sense. A fan's interpretation of the ending and why it seems like less of an asspull.

http://www.avclub.com/articles/skeletons-in-the-closetendgame,81693/

Korra’s just found out the best healer in the world can’t do anything for her. Her last hope to hang onto the identity she’s had since childhood is gone. Mako tells her he loves her; she tells him to go away, that she’s “not the Avatar anymore.” Her misery calls back to the dream she had in “The Voice In The Night,” where her own subconscious—dressed as scary Amon—tells her “Once I take your bending away, you will be nothing.” Korra goes out to a cliff, and walks up to the very, very edge—she’s close enough that from her looking-down POV we see a tear actually falling all the way down the side of the cliff, which means her head is leaning out over the drop. I can’t think of a reason to use that specific shot unless it’s to imply that she went up there to throw herself off the cliff.

Supporting this theory: When she pulls back, sits down, decides to keep living as a person who isn’t inherently special, and starts thinking for real about what that means—that’s the instant Aang finally shows up to declare “You have finally connected with your spiritual self!” This all feels like fan-wanking, but the alternate explanation seems to be “He showed up then because she was… continuing to be sad? And that continuing sadness somehow resolved her spiritual block?” That’s unsatisfying, unearned, and also doesn’t really make a lot of sense. The more I think about the oddness of that POV shot, Aang’s timing, and, most importantly, how not okay Nickelodeon would be with showing a hero contemplating suicide, the more sense it makes to consider the final-final battle of the season as completely internal—and pretty damn dark.

Tl;dr: Korra was going to jump off the cliff at the ending. She was contemplating suicide. That's what the lowest of your lows is, when you contemplate suicide. Just ask anyone who suffers depression. That's why Aang appeared when he did, and why her tear fell of the ledge of the cliff.

Not my idea, stuff from that website is quoted but once I realized it there's no doubt in my mind this is what happened. It makes the whole thing a lot more satisfying.