These can be very different reasons. This will certainly differ from person to person. What is normal for one person may already be a sign that more than normal gambling is going on for another. If you enter an addiction phase, you should try to get out of it as quickly as possible. Write down the symptons.
I don't think it would be very much possible for the person to get out of the addiction after getting into it without realizing it.
Most of the people who get into the addiction to gambling always chase down their losses till losing it all and repeating the same cycle again and again.
Useless someone close to watch their habits they will not be able to get out of it.
With that attitude a person would continue to lose their money, cause even they win in some point greed will come after, thinking that luck finally guiding them so instead of quitting and enjoy for a while they will continue and lose it all back to the house.
This kind of problem is very common to most gamblers. Without proper balance with their gambling habits, this sign will lead them to addictions.
Practice to stop and always in control of your activities is very important to manage beating this problem.