Lets drive away from all the closed minded definitions and try to properly define a 'homeless' individual. The most common form of homeless is the addict; these people come from abusive families, relationships and lives of many different forms. These forms usually end up in shelters and the dark parts of outside due to addiction or mental illness.
In another direction you get the vagrants who use train hopping and hitch hiking as a means of getting around, these types are very similar to the above and can often get into more trouble(they range from street punks to low life backwash.)
On the brighter side of this life you get the few odd types who chose ancient survival methods over modern day survival. These types typically do not stay in shelters or the sidewalks and are far more difficult to locate due to the fact that many of them do a lot of walking across country. These usually range from odd aboriginals trying to live out their traditions to the so called anarcho-primitivist movements. Many of these types vary in their survival methods. Very rarely do you find one that knows everything on how to survive like our ancestors did.
Not all homeless are depressed, insane or trash you know...