What explains the disparity of outcome in people who join the military?

American culture seems to be rife with men who went to the Marines and after being discharged of duty went on to either lead successful lives or who's life took a turn for the worse and ended up on the street.

Of c, the two groups are not equal in numbers and the third much larger group lies in between these two groups. Now, I still am interested in the disparity between the extremes. Why do some people who join the Marines go on to create an over represent the Marines amount the successful, while others end up on the street? They are all given a clean slate somewhat and are exposed to the exact same environment, what do the successful learn which the unsuccessful don't?