In the context of substance abuse, it's not hard to see the original implication of the now-largely meaningless aphorism: People trying to beat an addiction have to work very hard to keep from falling back into the habit. An addicted person will often find him or herself doing the same things (hanging out with the same enablers, doing the same activities that surround substance abuse) and expecting things will be "different this time."

Thus the admonition that someone is "insane" if they expect change when they are doing the very same things they did when they were dependent.