The reasons why people use drugs morph over time. For most I'd presume that the original reasons for using are relegated behind a new set of reasons that have developed as a result of using.. rationalizations basically. Also when people tell you why they supposedly use you do not know whether it is them talking or the drug/urge to use that is talking. They might say they know what they're doing and that it's all kosher.. but 20 years down the line they might sober up and look back and say "nah.. I was kidding myself. I was wrong".. they realize the drug used them, not the other way around.
I started with alcohol at 16 (social pressure), then cannabis which morphed into a several year long habit. I took mushrooms, LSD, and DMT during this time (and other random drugs on occassion), but my use was sporadic and infrequent (except for cannabis). I didn't have the confidence to turn away from it all because I wasn't very social or developed and couldn't hold my own.. I needed to fit in and develop. I wouldn't change the way it happened.. I've learnt a lot.. but also made a few mistakes. But looking back on my cannabis habit I can see my reason for using it changed over time.. at first it was fun, novel, had a laugh with friends, but then it became habitual and I did because I was addicted to it and didn't want to grow up.
I'm now sober (25yr old) and have no desire to return to ANY of them. I realized that actually I LOVE being sober, being clear and focused, and that the problem is not so much with me but with a world that sucks fucking balls and that I need to forget caring so much about society (it's fucked) and just do the best that I can for myself and those I can actually touch (family, friends, locals etc).