There are many reasons why people become homeless, including:
Lack of affordable housing
Unemployment
Poverty
Mental illness
Substance abuse
Domestic violence
Job loss
Foreclosure
Post-traumatic stress
Relationship breakdown
Lack of qualifications
Family background issues
A traumatic event
The inability to pay for needed health-care
A criminal background that got in the way of finding a job
There are also individual factors that can lead to homelessness, such as a lack of qualifications, relationship breakdown, or substance abuse.