I enjoy riding up mountain passes. I like the scenery and I like the challenge. I'm not a particularly fast cyclist on the flats, but I'm a fairly strong climber. Like others have said, there's a significant psychological element to climbing. The gradient and variability of the gradient are important, too. For example, my favorite pass in North America is Logan Pass on Going-to-the-Sun Road in Glacier National Park in Montana. Riding west to east, the grade is fairly constant and never excessive. Also, the scenery changes during the climb and becomes more spectacular as you go up. As you climb higher, more and more surrounding peaks become visible, and frequently additional wildlife, too. Contrasted to that pass is the famous climb up to Alpe d'Huez in the French Alps. I found that climb brutal. The grade varied significantly throughout the climb, there were several extremely steep sections, and one of the steepest sections was at the very beginning out of Bourg d'Oisans.