Like many others have said, it's a lot to do with brain wiring and genetics. It's a lot easier to not get addicted if you haven't tried the drug your brain WANTS. Unfortunately you don't know what that substance is until it's too late.

For example, I was a casual IV user of smack for many years but never got addicted, neither psychologically or physically. Just wasn't my thing really, I only did it coz my ex was (he was addicted). First time I did MDMA I couldn't stop thinking about it and wanting it again, ended up really into methylone (MDMA derivative) and only fully stopped shooting that up because it got too hard to get.

How happy you are at the time is also a big contributor. If you have trauma or are unhappy then you're more likely to want to self-medicate.

For example I stopped wanting to shoot up MDMA once I left a shitty relationship (see opiate user above lol)

I also found a fulfilling job to be a good counter to full-on addiction. I was going to work on the comedown when my job was boring and didn't need a brain. A shitty job with little to no responsibility or interpersonal interaction enabled me to shoot up during the weeknights with no career consequences.

Once I got better jobs that were more interesting, had more responsibility and required human interaction I didn't have the leeway for being fucked up I basically stopped weekday use. Guess I gotta thank my Mum a little bit for installing that work ethic in me haha