In the year that I worked with the homeless, seeing 10-16 a day, 5 days a week, I ran into 2-3 hard luck, circumstances actually screwed them over stories. One that let alcohol get the better of them, but that one had a good ending that the individual worked hard to overcome and DID. All the rest were drugs mostly. There were also some serious mental illness, but not the majority. The majority were drugs and alcohol.