I'm reminded of a sad story about a kid, probably 10ish, who overdosed on prescription meds designed to help him not sleep so deeply (he had problems with bedwetting because his bladder wasn't noisy enough to wake him).

Failing to understand the purpose of the pills, he decided that if he just took all of them, his problem would go away.

Granted, adults (usually) have better judgment than a 10-year-old about things like that, but nonetheless patients almost inevitably are able to make better decisions when they understand why they're being asked to do things.