Well, it took nearly 30 years to figure it out. But I have. And I am very glad for having discovered why.
Homelessness is never caused by just one thing, but always a combination of events and situations. It is the proverbial "perfect storm."
For me, the storm began at birth with a combination birth defects. The lessor was problem was that I have a small but noticeable amount of scoliosis, curvature of the spin. The more problematic "defect" (although the word "defect" is most likely the wrong description, I use it for expedience) is that I have a condition called Asperger's Syndrome. Asperger's is now classified as a type of high functioning autism. But it was all but unknown back when I was born. So, the problems I had that were a result of Asperger's were misunderstood by my family. They believed that the problems I was having, the difficulties I was having in school, and the difficulties I had socializing with people were due to flaws in my character. I had a hard time focusing on things like school work, and I failed miserably at activities like sports. So, my parents accused me of being undisciplined, and lazy. I knew those labels were inaccurate, and did not address the real source of my problems. Because of my Asperger's, I could not communicate to my parents, or anyone else, what was really going on. I had no vocabulary to express what I was feeling. So, because my parents incorrectly diagnosed my problem, the course of action they took to "correct" my problems was also the wrong thing to do, it only exacerbated my problems. Instead of being empathetic, understanding and encouraging, my parents set out on a course of long term punishments, and chastisments, criticizing every thing I did, thinking that doing so would inspire me to be better. Since children naturally desire their parents approval, that tactic might have some positive effect. But my parents did not use this tactic correctly. They fell into a pattern that still exists today. No matter what I did, no matter how well I did it, my parents looked only on the negative aspects and criticized me for them, never giving recognition to the good things I had done. This had some very negative effects that continue with me today. Having Asperger's alone will make a person depressed, but with the added continue disapproval of my parents only added to my depression. It also caused me to develop a belief that I could not do anything right, and that every thing that went wrong was my fault.
So, with the combination of struggling with Aperger's, and lacking in any confidence, lacking any belief in my abilities to succeed, having absolutely no moral support from family, and having no friends due to Aspergers, suffering from depression to the extent of attempting suicide on more than one occasion, there really was no other course for my life except to become homeless. All it took was the last little push by my family, to leave the house (they decided that when I turned 21 I would no longer be allowed to live at home), and so within a couple months of attempting and failing badly, at living on my own, I was homeless.
There was one chance of bonding with my father, and that would have been through sports, as my other brother had. But because of my spinal deformity, and the clumsiness that is part of Aspergers, I did not succeed at sports either. Because of my ability to perform satisfactorily at any task that was important to my father, he emotionally abandoned me at an early age. My mother had emotional issues of her own, suffering from depression herself, to be of any help. She refused to discuss any topic of importance beyond the weather. Additionally, my parents, especially my mother, saw any kind of expressed opinion, or disagreement, as an act of disrespect on my part. So I was discouraged from expressing myself, especially of matter of importance to myself.
I remember, of all the competitive things my father involved himself with, he was lousy at the game of chess. And when at the age of 14, I beat him at the game for the first time, he was stunned. But instead of congratulating me for my success, he said nothing, and never would play chess with me again. This taught me a very bad lesson about the cost of being successful. I was punished for not being successful, I was punished for being successful. Is it any surprise that I've been fairly unsuccessful in life? I still carry a lot of resentment towards them for the way they treated me, and raised me. It is a heavy burden.
With every case of homelessness you will find a combination of a personal problem and a negative reaction to that problem from friends and family. It is never just drug addiction, or just mental illness that causes homelessness. But throw in a heavy dose of reject and mistreatment for those problems, and you will have a homeless person.