In my opinion I believe a large part of happiness is derived from solving problems. 

We as humans are hard wired to want to solve complex problems. 

Working my way from the beginning, in order to solve complex issues one needs to have mastery over a topic. Only when we have mastery can we begin to delve into the nuances of a subject or topic.

Example: You can't work on a pin-down catch and shoot 3-point shot if you can't even make one from a stand-still position. 

A less sport specific example would be job satisfaction. Let's say you had a boring data entry job. Except you were THE BEST data entry person in the county, you may actually enjoy high levels of job satisfaction performing a task many would consider to be as entertaining as watching paint dry. In fact, the mere fact that you are considered the best at this job is what may motivate you to want to do better. 

As we continue to get better and hone our craft, we begin dealing with larger more complex issues. This is the trainer who wants to learn not only why their shoulder hurts, but what mechanisms occur that causes their shoulder to hurt and how to fix it. Some of the most gratifying clients I've worked with were the ones who had problems that were deemed "outside of the scope of practice" for another trainer when in fact it was merely outside the scope of their knowledge. 

See its not finding the dream job or attaining a certain level of wealth that motivates us, it's the ability to find work that we deem to be worthy of our time. Work that we want to work on, work that we are appreciated for.

And this only comes after years of hard work whether it be shooting baskets or punching in numbers on a computer. Work is the variable of success.