Ehh? Wasn't Rush's motivation pain? Isn't the skanky crack motivated by excape from the responsibility for his miserable life? IMO this is a huge differance. Once hooked the motivation is once again the same, avoidance of the pain of withdrawal. But the skanky crack addict will take a larger dose to also get high.
That's really what I was referring to. Everyone gets in for different reasons, but once you're in, the motivation is always the same - to get high.