Can you fix my roommate?
No, but I might be able to show you how to "fix" your relationship with your roommate. Or at least improve it.
This isn't just wordplay or quibbling over semantics, either. And I am well aware of how obnoxious and difficult some people's behavior can be, so I'm not denying unpleasant realities or living in some kind of dream world where real problems don't exist.
I am convinced that labels -- "He's just no good," "She's evil," "My roommate is just a slob" -- are just irrelevant. Are they factually true, at least some of the time? Maybe.
But they just don't do any good. The goal is not to change someone -- it's usually impossible, anyway -- the goal is to improve the way you deal with each other.
This perspective has a better chance -- not a guarantee, but a better chance -- of succeeding as it avoids much of the defensiveness that occurs when people feel that they are being attacked.
A good parallel is the endless "flame wars" that occur in online forums when people insist on arguing about "WHO is right." Those discussions that focus, instead, on "WHAT is right" have a much better chance of being productive.