Upton Sinclair has a famous quote that goes, "It is difficult to get a man to understand something, if his salary depends on his not understanding it." I see a similar principle at work with many of my high school students: It is difficult to get a student to understand something when their self-image depends on them not understanding it.
I have many perfectly intelligent and capable students who struggle in my classes, not because of a lack of intelligence or a learning disability, or even because of that old standby "laziness," but because the image of themselves they are trying to project conflicts with their learning.
Here's an example: A young man dreams of being a rap star and identifies strongly with that perceived lifestyle. Rap stars, however, are not competent at math or quantitative reasoning (at least in his mind), so he is not motivated to become competent in math or quantitative reasoning. In fact, he's actually motivated to not become competent in those areas, because he thinks that would make him less like a rap star.
Here's another example: A young woman wants to feel attractive and desirable to men and identifies strongly with beautiful women on TV and in music videos. These women are very beautiful but they are not particularly intelligent (at least in her mind), so she is not motivated to think of herself as intelligent. In fact, she actively embraces a lack of intelligence, thinking that this makes her better aligned with her idea of the media-fueled sex symbol.
There are many other examples, most of which are similarly stereotyped: kids see themselves as athlete, thug, pothead, cheerleader, drug dealer, cool kid, burnout, or any of a number of standard teenage fantasy roles. And if they happen to be embracing that role strongly, and if they've decided that learning, or being a good student, or being intelligent, doesn't fit with that role, it's tough to convince them otherwise.
Image by Gueorgui via flickr.