I've been having the same thoughts watching my ANephew spiraling down, further than ever having 'upgraded' from prescription opiates to heroin or so I've heard - but with his behavior lately it's probably true.
How can he do this, make this choice? He watched his father's addiction mess up the family and die way too young because of drugs (not from them, but someone under the influence literally sank the boat). AN was only 8 when his dad died, so he was too young to see that drugs stole his father, but after a couple of rehabs he's been educated as to this stuff.
He went to rehab and after his mother told me "I had my sweet boy back." Who knows why anyone picks up again, it's definitely a choice.
The easy choice is to immediately numb the craziness going on inside your head by using.
The hard choice is to learn how to get through an episode of anxiety or regret or depression or fear without using. I made the hard choice and it took forever, but I no longer shriek at sudden noises, can hold down a job longer than four months, those are two major things I could not do before I started talking type therapy.
It's not only our family, I've seen so many addicted parents with addicted children and then things get really crazy... My brothers chose addiction, I ended up supremely messed up (PTSD, yada yada...) and struggle with alcohol but mostly I'm sober, except for antidepressants but those keep me alive.
I was in my 20s in the '80s, and I was offered plenty of coke, but I'd already seen what drugs and alcohol were doing to the family AND I already knew I was messed up and didn't really want to get more messed up so I always just said no. I'm glad for that.