I'm not an addict, but I have been on opiates for pain conditions on and off. They make me feel good, the euphoria is nice, I become a nicer version of myself, the pain of the world feels more distant, and I feel like I can do anything. It's similar to some other drugs in that way except the pain part. It eliminates all pain. Nothing bothers me anymore.
But then when it wears off, I can become an asshole from hell. Irritable, sweating, nervous, reactive... and this is why I won't get much more involved in opiates as a DOC. I wasn't even on high doses either.
In this, I could see hints of why a more serious habit could ruin someone's life. Functional addicts are real but without safe support systems I don't see how they can remain at the same functionality level forever. How many people have the will power to not take just a bit more, especially if their lives are unbearable?
People in Switzerland are lucky... but most of the world has still drunk the war on drugs koolaid thanks to the United Nations / USA.