People have trouble understanding OO because usually it is presented informally with insufferable dog/cat/mouse examples.

Moreover, the fact that "subclassing==subtyping is unsound" is swept under the rug. A lot of people actually grasp the latter at an intuitive level, that's why they're never comfortable with objects.

OO should be taught as abstractly as possible (I recommend Didier Remy's writings). After grasping the concepts, many people will conclude that they'll be better off with FP and modules.