Throwaway account for obvious reasons.
When I was a kid I knew I would drink, from about the age of 8. I don't know why, I just knew - although I didn't start till I was around 13. I never thought I would smoke, even though most of the adults I knew did smoke. As for why I started - I was at a party, around 14 years old, and the host's parents locked the booze away - so I thought "I'll do this instead".
In my experience (and I'm no expert and I'd never want to put words into anyone else's mouth), addiction is a mixture of different factors. There's why you started in the first place, there's a psychological need that is going unfulfilled, there's habit and there's actual physical addiction.
For me:
LSD - not a problem. I had a great night out, thought "that will never be bettered" so never took it again.
MDMA/Ecstasy - again, not a problem. It was entirely situational - go out clubbing, take some pills - and when I stopped clubbing I stopped taking it.
Cannabis - I smoked weed constantly for around 10 years, often rolling one up last thing at night so it was the first thing I did in the morning. I stopped when I actually had to start working for a living and I couldn't afford the lack of time it caused. So I would say it was basically habit and boredom that kept me at it.
Drinking - I used to drink heavily, daily, for many many years. I was never a "hide the vodka in the toilet cistern" type, I just would go out and have a drink and not stop till I fell asleep. Eventually a combination of health issues and being fed up with wondering if I'd made a fool of myself last night made me try to stop. And it was HARD. I never had serious physical symptoms, but alcohol is everywhere. It's changed a bit now, but buying soft drinks when out was frowned upon and so many of my routines involved it. I stopped with the help of reddit (r/stopdrinking is the most wonderful place on earth full of kind and supportive people) and lots of fizzy drinks and ice cream (and yes, I put on weight, but I figured one issue at a time, and now I've lost that weight and am much healthier all round). Definitely habit, social/psychological and physical.
Cocaine - 2 spells using cocaine heavily. Once I used to use it to get me out of bed in the morning, but it was easy to stop. Second time I used it every day for around 18 months. I think this was because I was in a bad way mentally and I just needed to silence the noise in my head. It's actually very difficult to take cocaine like that - it doesn't last long, it hurts, your nose is constantly blocked and it's a pain to prepare. For me, psychological void.
Nicotine - the hardest of the lot to stop. Sort of. I read the Allen Carr book and stopped smoking 20 per day almost immediately. But I just couldn't stop smoking when I went out - it was intrinsically linked with "having a good time" (see why I started). Like cocaine, it's short-lived, but it's so easy to "top up" again, so you gain a series of little "boosts" throughout the day. Of course, you need the boosts because your brain stops functioning as it wears off. So it builds habits and has a physical effect. In the end I got hypnotised which helped me stop when going out - but I still often ask friends if I can hold their cigarettes, just so I've got something in my fingers. This is a mix of all of the factors combined.
Caffeine - never been an issue for me - I feel like it has no effect whatsoever. But the people who say "don't speak to me before I've had some coffee" certainly sound like I did when I was using cocaine.
I would say in none of the cases were "effort" and "willpower" part of what made me stop - I had to figure out why it was happening and deal with the underlying causes.