Hi Michael B,
The problem with confident people in general is that you cannot predict their potential for depression. Many apparently confident people may suddenly become depressed. Something may have triggered their pre-existing Depression to the surface. The same with addiction: people who use drugs for he first time may experience a "high" for the first tie in their life. They were not aware that in fact they were depressed all along. Healthy people may have a bad experience using drug for the first time and don't return to them, as they cannot get a high from that.
People usually get hooked on drugs because it is an anti-dote to their Depression (anxiety whatever) of which they may not bee aware.