Addiction can involve many things, not just drugs, alcohol, pornography and gambling.
Some people become addicted because of their pain, both physical and mental pain; they use drugs or alcohol to get temporary relief.
Some people become addicted out of curiosity, or because it was the popularity at the time.
Some people get addicted due to emptiness; they need something to fill that void.
Some people get addicted because they are lured by addictive products or systems. For instance, social media companies design very addictive mechanisms for people using their platforms; gambling companies design highly addictive mechanisms that keeps people losing money.
Basically, addiction can be very personal, but it can also be a very profitable business.