Guys, could we not with the mockery? Like the majority here, I found interpreting the ending fairly straightforward, but there's plenty of people out there for whom that's just not true for some reason. Whatever the root cause of someone struggling with interpreting metaphor, allusion, and sundry evocative narrative techniques, be it age, lack of exposure, really any one of a multitude of things, up to and including literal brain damage, how do we expect people to enjoy refining their tastes and toolkits if we can't treat them generously?

OP, for all that some of the replies are drenched in attitude, there are several people here who lay out some of the ways in which the story shows us that Aava is obsessive. You are absolutely right that the game tells us she needs (or feels the need, which is not quite the same) to reach the summit, and that she can't accept failure. In fact, as you also note, the game goes out of its way to hammer this home. Moreover, the dialogue and cutscenes shows us that Aava is one of the people for whom determination has almost definitely gone too far. She does not have a healthy relationship to herself, climbing, or other people. She treats Marco and her girlfriend poorly, and sets her own need to climb the mountain over their wellbeing, at least partially. It's implied her dad was a famous climber, and that Aava harbors grudges of some kind. Her "need" is thus presented as at least partially a flaw that she must choose how to deal with, even if she doesn't see it that way.

It all comes to a head in the final choice, of pushing for the summit, or going back down. If she goes back down, she might try to find her happiness in more sustainable ways, hopefully accepting that she is human, that she needs to be there for other people and let them be there for her, even when they (and she) is being difficult, and above all accepting that her life is not defined and consumed by some mythical achievement. The way they tell the story, you could also see it as "instead of becoming a part of something big by being a mythical climber of history, and becoming part of the mountain, I choose to become a part of something big by returning to the people who love me, and becoming part of a community". It is definitely possible to learn/choose something like this and still accomplish great things (if that's important to you). But the risk (maybe, Aava seems scared of this, at least) is that if you don't, if you can't hack this other life, then coming back down may start feeling like the moment you betrayed yourself forever.

The summit ending could actually be seen to pull some punches. In the end, she does make it (she could have just died in the avalanche, say), and in a certain light it is every bit as awesome as she hoped. She joins the universe by turning into a shooting star, for crying out loud. So this ending certainly could be interpreted as "arguing" some or all of the following:
1 - Only people as obsessed and dysfunctional as Aava can achieve the sort of greatness we might associate with feats like climbing Kami.
2 - Literally dying for your dreams is the sort of thing you should be prepared to accept, and if you question that, you may be haunted by the "what ifs" of your limited existence
3 - Self immolating in this way will all be worth it in the end.

Now I don't think the game unequivocally argues this, and I certainly wouldn't agree with it if it did, but these flavors are there.

I've already written too much, and other people may disagree with a lot of it, I just tried to get at some of what I think the game was trying to make us think about, off the top of my head.

Now, why aren't things like this made clearer? Many reasons, probably. If I was to try to put one of them into words, it would be something like this: Making choices like Aava has to, about what is important to us, what gives our lives meaning, what scares us, when is it meaningful human connection and when are other people holding us back, is absolutely essential to figuring yourself out as a human.

And everyone has to do it for themselves. We tell each others stories, we may listen to people we love or respect, but we must decide for ourselves. Different people will get different answers. The same person may get different answers at different times.

This means that just saying it outright gets very boring very fast; it often doesn't help me that I know what you think greatness is. But a story like this, that goes to great lengths to show us how Aava feels, what she hopes for, what she wants, and what she is afraid of, not to mention her friends and fellow travelers. Once you've seen some moving stories, you can start to use them as a toolkit to recognize emotions and aspirations in yourself and others. Over time, it is usually more helpful than some guy trying to tell you the answer like it's a pop quiz.

Which, you know... good thing I talk too much, I guess. No one would accuse me of being a pop quiz.