It’s a combination.
You are a combination of chemicals: a brain.
taking drugs gives you the same sense of enjoyment eating does.
What is fun about eating? Nothing. You don’t decide to eat if you’re full, it feels gross.
You dont constantly wonder if you should eat, you just suddenly hunger.
Technically eating is a choice, it just requires a lot of willpower to starve. They don’t decide that they want the drug, they hunger.