Opioids are actually much less harmful than other stuff, even alcohol. Alcohol is a neurotoxin, i.e. it literally kills brain cells. On the other hand, everyone's bodies produces opioids naturally. Endorphins are opioids, and the rush you get when you work out at a gym, is you getting mildly high on the natural opioid, endorphin (whose name is short for "endogenous morphine").

Synthetic and natural opioids have the same mode of operation on your brain as endorphins. They bind to these things called "opioid receptors"[1] in your brain. There is a biological purpose for the existence of these receptors. Quoted from Wikipedia: "The endogenous opioid system is thought to be important in mediating complex social behaviors involved in the formation of stable, emotionally committed relationships."

Opiates actually seem to have a significant benefit to people suffering from severe and refractory major depression, where all other legal anti-depression medication has not helped them.[2] Now compare this with alcohol (ethanol) -- a substance that recklessly goes around destroying cells in your brain and your liver, which people take just to get rid of some social anxiety. Huh.

[1] https://en.wikipedia.org/wiki/Opioid_receptor

[2] http://www.opioids.com/antidepressant/opiate.html