Almost everyone living on the streets is there because of a series of choices which they made, usually over a long time frame. Perhaps at the time they were not entirely voluntary choices, or conscious choices, but cumulatively they have chosen a certain pathway that led them to losing their income, job, belongings, and housing. Even an abused wife who's been dumped out by her husband, well, she did choose that husband.
Obviously there is nobody who rises from a warm bed one morning, says "screw all this!" sells all they have and goes to rough it on the streets (except, perhaps, for Saint Francis of Assisi, but he found something productive to do.)
I spent a lot of time Downtown hanging out with minors, and one day my good friend who's a dad confided to me something surprising. He said "Most of these kids aren't homeless or runaways, you know. Most of them are from wealthy neighborhoods and have good parents, but they come out here just to mingle." So it was like Future Runaways of America out there. And make no mistake, the area has a huge effort of outreach to runaways and homeless minors.
Once someone winds up on the streets, more choices are offered. Those choices can either drive a person deeply into a hole from which they will never recover and never get off the streets, or they could just keep them going while they can stabilize and find needed services, and begin a process to get off the streets. Many people will pursue the former. Even if they pursue the latter course, they may not make it. And then you have people who realize, hey, this life is ok for me. I guess I don't mind it so much, and it's not worth trying to get housed. So they stay as-is for the rest of their lives.