People generally don't take a few math classes and expect to become a mathematician or some physics classes and expect to become a physician.  At some point, they realize they reach their limitation and will tell themselves "I have enough of that" and move on.  This is not true for language learners.  This group is a special bunch.  After a couple of years of learning a language, they usually get upset when they don't improve any further because invariably their expectation is to become a native.  Anything less won't suffice.  For some odd reasons, the difficulty of mastering a language does not exist like it does in the other fields of study. If I were a language, I would be offended because people think I am so easy.

By the way, that physics classes and physician bit is an attempt at humor.