This is really difficult question, and I do not think you will get a good answer if you do not listen to two sets of people:
1. People who have suffered from a serious gambling addiction and recovered (so they have some insight into their problem)
2. Psychologists who have researched and/or treated people with serious gambling addiction

For most of us, who are not these people above, it is just guessing.
But... if I am allowed to guess on this thread (and I think that I am ) then I guess that it is a mixture of two things:
(i) being psychologically more likely to develop addictive behaviour
(ii) having difficulties in your life (economy, family, work issues, etc) that trigger this behaviour

One thing I know - it is a terrible and destructive thing to see happening to a person.

(a) pico-dasoi
(b) 3NpGUXQT3rkBAgVfwbYEkzgG7ybQw4JFKK
(c)
(d) VALIDATE

welcome! nice!