High School Dropout Epidemic

I really, honestly do not understand High School dropouts. There are SO MANY doors in our society that are ONLY open to those with a diploma, why would you give it up if you had any other choice? But this article suggests that the rate of quitters is way up and shows no sign of letting up any time soon.

A recent study by the Department of Education found that 31 percent of American students were dropping out or failing to graduate in the nation's largest 100 public school districts.
The implications from dropping out of high school are enormous, including a higher risk of poverty and even an abbreviated life span.

I wonder how much of this has to do with the No Child Left Behind tests, which have ended the practice of social promotion by providing clear standards for achievement? It may well be that this issue has been there for years, but we are only knowing about it now because school has become more difficult (and correctly so). Any educators out there with an opinion to share?