Disability, especially where disability services are nonexistent or poor performing.
Lack of accessible healthcare. People who have some kind of chronic and weakening disease but cannot get healthcare either because they don't have money to afford it or because the government will not give it to them are simply too weak to go and work every day so they remain poor and homeless.
Mortgage foreclosures where mortgage holders see the best solution to a loan default is to take and sell the house to pay off the debt. The popular press made an issue of this in 2008; the real magnitude of the problem is undocumented
Poverty, caused by many factors including unemployment and underemployment.
Relationship breakdown, particularly in relation to young people and their parents
Mental disorder, where mental health services are unavailable or difficult to access or as a result of deinstitutionalization. A United States Federal survey done in 2005 indicated that at least one-third of homeless men and women have serious psychiatric disorders or problems.
Substance abuse can cause homelessness from behavioral patterns associated with addiction that alienate an addicted individual's family and friends who could otherwise provide support during difficult economic times.
Forced eviction – In many countries, people lose their homes by government order to make way for newer upscale high rise buildings, roadways, and other governmental needs. The compensation may be minimal, in which case the former occupants cannot find appropriate new housing and become homeless.
Lack of affordable housing. An article in the November 2007 issue of Atlantic Monthly reported on a study of the cost of obtaining the 'right to build' (i.e. a building permit, red tape, bureaucracy, etc.) in different U.S. cities. The 'right to build' cost does not include the cost of the land or the cost of constructing the house. The study was conducted by Harvard economists Edward Glaeser and Kristina Tobio. According to the chart accompanying the article, the cost of obtaining the 'right to build' adds approximately $600,000 to the cost of each new house that is built in San Francisco.