One thing I’d like to point out is that the people you see on a regular basis on the streets are typically chronically homeless. The temporarily homeless are much less visible (because they’re around for less time and because they tend to make more use of the available resources, shelters and whatnot). And those are two different populations, with different situations and problems.
The chronically homeless usually have multiple problems, and homelessness is a symptom or a result, not a starting point - they may be addicts, victims of domestic violence, mentally ill, etc.; sometimes they are all of the above and more. Some people have so many problems that it’s like unpicking a tightly tied, complex knot; you try to work on this part, and this other part becomes worse. You try to start at this end, and you find you need to move something else first. These folks are the ones who are likely to stay in their bad life situations (sorry for the jargon) longer - because so much work has to be done to get anywhere. And sometimes they aren’t, for one reason or another, able to do or committed to doing that work - it takes an awful lot of effort, and these kinds of problems can really sap your energy, not to mention screwing with your organizational and orientational skills. So if these people are homeless, they’re going to be homeless for a while, and you’ll see them more often and notice them more.
The temporarily homeless, like MsRobyn describes, are a widely varied group. You can end up temporarily homeless for all kinds of reasons; one of the most terrifying things for me, when listening to the stories of folks who have ended up homeless, is realizing that something similar could have happened to me. I am lucky in that I have a safety net, in the form of people who care about me and who are able to help me - without those, I’d have spent much more time than I did homeless. (I spent a very short period of time homeless as a teenager.)
Since the temporarily homeless can become chronically homeless - homelessness makes you much more vulnerable to other kinds of problems - resources are often directed at emergency intervention, which just means taking care of people as soon as they become homeless. People who have been homeless longer require more resources and have less available for them, on the whole, so once you’re that far down, it’s really hard to get up.
That said, yes, some people choose to be homeless. Sometimes they’re chosing a lifestyle, sometimes they’re chosing the lesser of the evils, sometimes they aren’t aware they’re choosing anything until it’s too late.
And some people who are homeless never had any direct choice.
You can’t know. And there is no single label that fits all homeless people. Just as with any other non-selected population, it’s impossible to sum up - or dismiss - the homeless with a single sentence. Try thinking of a single sentence that does not describe our preselected traits (by preselected, I mean that, for example, we all have to have some kind of computer access to be here, so our presence here means we’ve preselected for that trait) that would apply to all dopers, and you’ll see what I mean.
(My answer is based on the following experiences. I used to work for an agency that dealt with a large number of homeless people, substance abusers, victims of domestic violence, and perps of dv. (Needless to say, there’s a significant amount of overlap between the groups.) As a writer and researcher, I now study high-risk ppns. And, of course, I’ve been there - very briefly, but scarily nonetheless.)