Let's take alcoholism for example.
The first sip is a choice. So is the second and the third.
But consider what alcohol does. It's a depressant. It inhibits your thinking. And that's the romance of it.
Alcohol alone is not addicting. However, when you have demons that you wish to escape from, alcohol can help you escape. It starts that simply and innocently enough. But, you can build an intolerance to alcohol. Soon, it takes two beers, then three, then five to hush the demons,
Should you realize then that you have a problem, you can try to quit. Except, alcohol withdraw is a thing. It's obviously not as bad as certain drugs, but it's well documented. And can be life threatening. So what then? Admit you have a problem? Or do the one thing that you know will make anxiety and shakiness go away and hush those demons just that much longer? Drink more.
At some point it's not as simple as drink or don't drink. And that's why it's considered a disease.
However, to broadly answer your question, addiction is a psychological disease. Certain people are more likely to be predisposed for certain addictions and studies show that addiction can be genetic.