From what I understand, the addiction mechanism for heroin is more subtle then non users think. Its easy to try once, enjoy it, and then not be compelled to do it again. The problem is many users see the lack of negative experiences on their first use as sign that the drug is "safe", so they use a month later, and it still ok, so they quickly start to use most weekends, ramping up until they use every day.
Similar to you, an old boss of mine well experienced with drugs told me the only two drugs he would advise people against are heroin and crack. Heroin because you're always going to remember your first time and its amazing, and crack because you will be amazingly high for 10 minutes, crash, and really feel the insanely strong need to take another hit.
so you can trust me when i say there is no such thing as a "if you do it once youre hooked forever" chemical or certainly not in the 100+ unique psychoactive chemicals i have tried.
feel free to ask any questions you may have