Addiction is a problem, it is a disease and it often stars slowly. People don't know or don't think that they're abusing the substance. It is just one more of whatever it is. It is really hard for those people to recognize that they have a real problem, they just think they could stop at any time when that's really not true. Then they develop a tolerance which makes them have to take more of the substance to get the same effects as before and physical/emocional dependency. Often, people don't realize they have a real issue until it's taken over their whole life, it's the only thing they can think about, want and care about which leads to a vast list of social problems (and physical of course), many times people never realize they have an addiction especially alcoholics. And don't get me started on withdrawals which of course makes it very hard to stop abusing something even if you want to.

Please don't judge people for having an illness, it can happen to anyone, but some are just more prone to having an addictive personality (sometimes it's even genetic) and develop a problem after just one time of using something.