It's much more tempting to seek out another hit if you have nothing better to do that day.
I tried to smoke weed a few times. I didn't just dislike it but downright hated it every single time. I tried a few other drugs, and responses varied from "I don't really like this" to "meh, so this is it, is it?" Never tried heroin, so I can't speak to that specifically.
Point being: individual responses to drugs vary greatly. I wouldn't say I'm "immune" to becoming an addict, but it'll sure take a lot more than for some other people.
Some people love opiates. Personally, I hate the feel of opiates. My brain feels like it's encased in concrete. This is not a good feeling to me.
I consider myself fortunate that I don't like the feel of opiates as it means that I didn't become addicted to them from the times I needed them medically.
I will, however, point out that musicians almost universally warn other musicians not to do heroin even once. Musicians aren't exactly straight-laced, and, if they're warning you about something, you probably ought to listen.
I am an anxious person prone to depression. In one especially bad period of my life I was prescribed Lexaurin (anxiolytic used to handle panic attacks). Taking that was a shocking experience - it made me feel calm, optimistic... and maybe for the first time in my life it made me aware of the ever-present baseline of anxiety that I was living with, like always. Lexaurin made me feel not anxious at all and it felt awesome. I kept asking myself - is that how other people feel all the time? I would give anything to live like that. If I had free access to it all the time, it would be really hard to resist the temptation not to use it. At the same time I can imagine that for many people - like you maybe? - taking Lexaurin would do absolutely nothing, because they live on that anxiety-free baseline their normal lives.
Roughly 1-2% of people who try heroin once are done forever; many many try once or twice and aren’t. For cigarettes the rate is about 20-30% who become addicted
It seems pretty likely to me that at least some people really will inevitably go down that road from one dose. Maybe not as many as the hardcore Drug Warriors would like us to think, but at least a few. It's an odd blind spot for the hardcore Libertarians - some people just aren't physically capable of coping with it, and no amount of willpower on their parts will change that.
When I was ~12 I got nitrous in the dentist's chair.
I remember the thought going through my head: Wow, this is really cool.
However, I didn't have ready access to nitrous! If my parents had a whipped cream maker, would I have started huffing whip-its? Maybe, but at least they aren't addictive. It's not like I would have gone into withdrawl after going through a box.
I think the bigger issue is the physical dependency that opiates have. Once the drug is no longer the "new toy," but the physical dependency is there, it's much harder to stop once the novelty is gone.