Take a 12-year-old kid in middle school. Let's assume he is bright enough to do well, if he did the homework, etc. He goes home every day to a dirty house, where he spends hours by himself playing video games. His mother is at work earning an hourly wage, and when she gets home, she is bitter and sarcastic. His father is gone. There isn't enough money to pay the bills, so the phone, tv, etc. are constantly being turned on and off (i.e., general chaos).
At his suburban middle school, he has to endure the humiliation of being pushed toward lower-status "vocational" programs despite his (at least average) ability, because of his poor attitude, laziness and the fact that his family is known in his hometown as being somewhat "troubled."
It is only natural that a young person in such a situation might become disinclined to work hard, because he doesn't know what the point of "working hard" is. In his world, and the world of his family, "working hard" just means putting in more hours that enrich someone else. He hasn't been exposed to any other ways of life, and the well-to-do telling him that he's "lazy" probably won't serve to motivate.
Having said all that, there is nothing stopping this person from realizing the error of his ways as an adult and acting accordingly. Luckily, we have many examples of successful "late bloomers" available to us these days, but I think for many kids the damage will have been permanent.