The way addiction was explained to me is that when they come down off of a high and crash again, their mental issues set in; they get depressed about their lives, they are upset about this or that, and yes, they do have clinical mental issues that hit them harder when they are coming down from their high, so they decide to get high again so that they don't feel depressed anymore... or they self medicate, as marle called it.
My ras would say some things that made absolutely no sense before she went into recovery. Depending on what he is using, most substances alter the brain, so they do tend to affect the user's logical thinking skills, as well as their memory, and other things I'm sure. My sister has to take meds to be able to sleep at night... I'm sure it's because of anxiety.
Sorry if I rambled... I hope something out of what I just said helped.
*hugs*