It's true that the most addicted people initially had pre-existing mental conditions, if not their common sense should tell them that they cannot win back their losses as they keep on gambling to recover them back. It's a norm that a hard drug addict will gamble, because he will always love to gamble for profit in order to buy more drugs.
Teens are copy cats, they do whatever they see on tv or on social media without knowing the consequences attached to it. Some of them might want to do it for fun, but along the line, they become addicted they don't have anyone to help them.
Gambling influence and other activities that influences one have a pre motivation behind it, just as you rightly said that addictions is a product of underlying mental illness, same also can be viewed with influence, because a thing can only influence someone who is attracted to such thing, so anyone that get influenced by gambling is already pre motivated to gamble and just waiting for the right time to venture into it, so for that we can categorized them among those self determined victims.
Same with Teens in our environments and home, many of them are already motivated by the why they either see they parent or any elderly person gamble and are motivated to do same, before the get influence along the line to start gambling, so they can't say that it just a thing of sudden outcome, but of long term preparation and admirations.