From a coaching / personal training perspective, knowing how to challenge the athlete / client in front of you means trying to get inside their head and understand what makes that individual person tick - what motivates them to try harder. Is it pride? Anger? Shame? Possibly even greed? Or (egad) narcissism?
A person who doesn't have any of the seven deadly sins is truly content and doesn't really want anything. Such a person must be sublimely happy, so why would they bother to hire a personal trainer? They're already found contentment and happiness. The only reason I can think of such a happy person wanting a coach or personal trainer is because they are amused by the idea of it. They want to do something and they're doing it for fun*. I will come back to that idea later. See the * further below.
It is the person who is yearning for something who really needs help achieving it. They are struggling for whatever reasons to achieve a goal that is beyond their reach. Perhaps it is a very common goal, like losing weight, and they are struggling because of their diet and lack of exercise, but also because they lack willpower - and quite possibly other factors as well. This is a quite common goal. But why do they want to lose weight?
Knowing what motivates yourself, what the primary reason you decide to exercise - especially if it is on the spur of the moment and you get the sudden urge to exercise, think carefully on that experience. What was it precisely that caused you to get the urge to exercise?
People rarely get the urge to exercise because they are feeling charitable or humble - although it would be awesome if more people were so easily motivated. "Oh look, a marathon that is raising money for charity. I think I will start jogging regularly so I can help raise money." How is rare is it that people take up marathon running just for charity? Extremely unlikely. More likely they were already into running marathons and they saw one that was raising money for charity and wanted to join.
Back on topic, once you know what things motivate you - truly motivate you - then you can use those negative emotions to focus your mind on the activities you want to do and get better at. Over time those negative emotions will fade and be replaced by feelings that are more positive, and you will eventually be doing the activity hopefully because you just plain enjoy it.
Which brings me full circle back to the Asterisk...
For example, I enjoy writing - but sometimes I am suffering a bit from writer's block and I have to find inspiration to write. One way for me to combat that and find inspiration is to find something funny to write about. In this case it was because I was thinking about Star Wars and Darth Vader memes while I was writing. It not only gave me some inspiration, it helped motivate my writing.
So it isn't just for exercising. It could be for doing any number of activities you should probably be doing. It does not have to be yoga while dressed like Darth Vader, but hey, nobody is stopping you from having a little fun.