What do you think is the key ingredient and how do we know for sure that someone is an "addict"?

Many things in our lives are 'habit forming'.  If you are used to brushing your teeth before bed, if you don't one night, you might have trouble sleeping, just because the routine is broken.  That's 'habit forming'.

Addiction is worse than that.  It's when you can't stop something without some serious pain or discomfort or disruption in your life. 

I am addicted to caffeine.  Seriously physically addicted.  If I don't have my big mug of coffee in the morning, I get a headache around 10:00 and nothing will make it go away except caffeine.  I'll have trouble concentrating on what I'm doing, and I might not be in the best mood.  That's addiction.

But I could taper off, or I could just quit 'cold turkey' and I'd be okay in a couple of days.  (I think.  I've never tried.)  Someone addicted to opioids would have a harder time quitting.  They could even die if they just quit abruptly.  That's a real addiction.