Physical challenge. Mental challenge. Planning (love it when a plan comes together...). Camaraderie and teamwork. Weather is cool. Jokes are funnier after you bonk...
Sense of accomplishment ("It's not the mountains we conquer but ourselves." Sir Edmund Hillary). Goal is tangible and easily measured, unlike most things in our everyday life. Ramen tastes great when you're there. Food and beer taste better and colder when you're done. Life's simple pleasures are amplified.
Awe and inspiration and the requisite sense of wonder - "Eastward the dawn rose, ridge behind ridge into the morning, and vanished out of eyesight into guess; it was no more than a glimmer blending with the hem of the sky, but it spoke to them, out of the memory and old tales, of the high and distant mountains." JRR Tolkien