pretty much every authority figure in the world will tell you the horrors of drugs. how bad they are, how addictive they are, how marijuana, alcohol, etc will ruin your life, whatever. so your perception of drugs is that they are this evil force, and you can't figure out why someone would ever use it.
now, say you don't have a good home life, you're just bored, etc. whatever the reason, and you start hanging around people that use substances, even benign ones like marijuana or alcohol.
you have a good time and realize these aren't nightmare death drugs, and that a lot of the information you've been fed by your parents and DARE and whatever other authority figures is wrong.
you can get drunk and high responsibly enough, there are drugs like acid and shrooms you can take that have no lasting affect, you can do a little coke at a party and it's not a big deal at all, hell even the pills your pal Mike gets for his ADHD can be taken by anyone and they'll make you feel good, and the next day you're fine.
now say you find out one day that your friend Jeff uses heroin. He doesn't shoot up, because that's some crazy shit only addicts do, he just snorts a little bit or maybe smokes it. he's not an addict, he just does it on the weekends, he does well in school, he's a personable and fun guy, etc.
you figure what's the harm, you try a little bit, and it feels good. it doesnt feel like a turning point in your life, it doesnt feel like you just signed your life away, it's just a good high feeling. you make plans to smoke a little heroin again later next time Jeff is around.
What you don't know at the time, and in a lot of cases possibly can't know, is that every addiction story has a beginning, not every addict is the same, and that you can be a functional drug user but you're basically living on borrowed time before SOMETHING happens.
imo this is how it happens for a lot of people. you get yourself involved in a social atmosphere where drug use is available, you dont have the right education or perspective to understand the affects of the drugs, and you dont necessarily have the adult experience to know how things are going to end.
obviously there are lots of other cases, such as prescription opiate abuse, but this is what i've seen growing up