This is difficult to express, but that difficulty inherent to my process of describing my perspective to you is a motivator for me doing so. And that's where difficulty/challenge as a component of knowledge acquisition is important.
We memorize and recall within the limbic system. That's the same system that also facilitates our emotional and physical responses to stimuli. And as I see it (keep in mind I'm not a neuroscientist), increasing emotional activity alongside the task increases the overall allocation of physical energy to the limbic system. I believe this is the physical component that defines "flow", or the ability to be so immersed in an activity or series of activities that, as I am at this point in my writing, we forget time. (I need to go stir tea on the stove. One moment...)
Alright. Um, our ability to sustain enough engagement in something to optimize our retention of that activity is, I believe, critically dependent on emotion. Think of reading a law book. Now, think of reading a law book on accounting. Now, think of reading a law book on accounting while referencing the IRS.gov web site for available white papers. The further we venture into emotional detachment from the activity, the more willpower we need to engage, the more energy we deplete through continuous activation and reactivation as we doze off and have to return and restart a portion of the activity.
I agree though that the challenge has to be enjoyable. I'm not interested in learning a new programming language while doing a handstand. I am, however, interested in learning a new programming language if a boy I like also likes it and likes to talk about it, and I want a reason to talk to him more often. (I had a stint learning C++ for that reason. And it's still one of the best things I've done because it expanded my understanding of paradigms and language capabilities.)