Addiction to something happens because the sufferer feels comfortable, full of satisfaction (dopamine effect) when doing that. slots feature amazing animations and also great prizes easily if the player is lucky. I don't like slots but for lottery and soccer betting, I really love and feel addicted 
Just because it's easy to do doesn't mean you need to play it every day and that's what most people became after they suffered such addiction from playing slot machines. some of them are buried in debt because of these small matters that they could have avoided if they remained patient about their life. Slots are addictive because the environment where they put them has lots of people enjoying what they are doing at the moment but in reality, most of them are soon to suffer some serious depression when they lose everything they have in this tricky game.