[Non-academic opinion based on clinical experience]
Why 'or'?
It's BOTH!
Some substances, like heroin, are intrinsically addictive. That's why they are controlled drugs.
Other substances, like alcohol, are not always addictive. Many folk might drink some days but the abstain for ages. Other folk DO become addicted to alcohol. There's a biological basis for this in some alcoholics but others turn to drink because of circumstances.
Some folk get addicted to other harmful behaviours, like gambling. The 'addictive personality' is widely discussed by Learned People.