There's nothing better than throwing your body against something daunting, something that doesn't seem doable at first. Maybe that's why I keep looking for really long and tough climbs on the bike.
Mt. Haleakala in Maui, the San Juan Scenic Skyway in Colorado, even the Stelvio Pass through the Italian and Swiss Alps. I'm afraid every time, but it's always worth it in the end.
Seriously, what does it take to get a decent burrito after a hard climb?