classes

Why are the easiest classes always the hardest and the hardest classes always the easiest? Every time someone tells me a class is going to be hard I psych myself out and become super worried about how i'm going to do. And then when people say "yea! you should take this class its super easy" I always get crappy grades? I've come to two conclusions. One, that if I think the class is going to be really hard I pay more attention and therefore do better, and if I think the class will be a breeze I don't put much effort into it. OR that the harder classes also have the better teachers, therefore I learn more in class making it easier for me to do well over all. And the "easier" classes don't have very good professors, so I really need to study more for something that shouldn't be that hard to pass. Does anyone ever have this same thought process?