I have a friend, for instance, who likes chemistry, has done a lot of cooking, and enjoys extremely difficult platforming games (ones that require jumping with exact timing). His thesis is "precision." Something in him loves to work in very precise ways.
Another friend keeps his DVD's in like-new condition, has a lot of action figures, and plays those same sort of platforming games, often with the first guy. His thesis, though, is "physicality." He finds something important in the physical nature of objects.
I chose those two examples for a reason: they have that one element in common. Often we assume that people who do the same things are, themselves, similar. In reality, any number of personal theses can draw people to a particular job/hobby/cause. Some people make cakes because they love the taste of things, others because they love the aesthetics, still others because, for whatever reason, using an oven is incredibly fulfilling.
The problem is that most people never know they have a thesis, because they naturally assume that everyone thinks basically the same way that they do.
*My thesis is that I'm always trying to figure out the workings of things, sometimes to the point of oversimplification.