sense of scale

There’s no two ways about it — the valleys carved out by the rivers are just plain ol’ big, and, of course, so are the Himalayan mountains, too. But, I think, the thing that makes it a mountaineer’s playground is that the sense of scale lies to you. It gets to your head.
By way of comparison, maybe it’s instructive to think about places where your sense of scale matches the reality. You go to the Tahoe basin, for instance, and you look around, and you point at a peak on the horizon, and go “I could probably get there”, and you’d be right; it’d be a day’s work of hiking, and up you’ll go. Or you might find yourself in Zion National Park, and you’ll see a feature in front of you, and go “oh, that’s totally climbable”; if it’s not about to break in half, chances are good that someone has already put up a route on it.
The converse is true in other places, of course. When you go to the East side of the Sierras, you might stand on a peak, and point at another peak, and go “man, it would be an enormous pain in the ass to get there”, and you’d be right; there’s just so darn much terrain between you and the something else, and it’d be days of hiking, if not technical travel, to get to wherever it is that you’ve aimed your finger.
The Himalaya is special, though. You look up beside you, and just uphill of you, you see a friendly snowfield, a little ways down from the top of the valley you’re in. You get the urge to run up, pack a snowball, and throw it — maybe it’d be a half hour away. You glance down at the topo, and you discover very quickly that your eyes have fooled you: you’re sitting pretty in your village at eight thousand feet, but the top of that wall is easily a 14'er, as if it were no big deal.
Maybe that’s what keeps people coming back: the peaks of unimaginable prominence that just sit there there on the horizon, and pretend that they’re there for the taking -- or, maybe, just the way that the impossible suddenly feels human-scale.