I don't agree with this.
This is true for some cases, yes. But it's just as common if not more common for people to take drugs to mask pain. It's not about the drug being fun, but about the overwhelming pain of life being temporarily numbed.
Some drugs start off as fun before becoming something more severe. Others are never fun - they're just coping mechanisms.
Of course my views my be slight jaded by the BS programs like D.A.R.E spouted off during the 80s. "Drugs are bad" "Use drugs once and die" "This is your brain on drugs". This propaganda pretty much ended any realistic talk about legal and illegal drug use. Some drugs are fun. Some people have to cope with massive amounts of pain in their life. Others do drugs because of a genetic predisposition to addiction.
Now we're just left with a massive mess with bad outcomes for everybody.