The enjoyment of riding up hill is two fold. One, it's a condensed cardio workout. That's pretty basic. Two, it allows one to refine skill sets by tackling harder and harder terrain.
Mostly that's about learning to handle and balance at low speeds and how to approach obstacles. Riding down over a root system is very different than riding up one. Negotiating steep switchbacks, powering through the turns... stuff like that. The harder the up trail, the more technical it becomes but in a slow speed, balance sort of way.
My after work rides are usually up for about 45-60 minutes on a trail that has a large number of challenges and 15 or so minutes down a harder route. Each way, each day, I try to either hit more obstacles or hit them in a better way.