I going to reprint a post here that I made on Absolute Write. The subject was a discussion of why conflict doesn't always have to be good versus evil. I answer as follows:
"I'll make the pitch for the case of Good v. Evil if no one else wants to. Lookit, conflict is defined as two irreconcilable wills, each trying to impose itself on the other. Your reader is going to identify with one side or the other. (And as a side note, you'll want to be in control of who they choose. Otherwise you have the book I'm reading now, Children of Chaos by Dave Duncan, in which I'm starting to root for the bad guys because I really don't care if the good guys win.) Once that identification is made, someone becomes good and, again by definition, someone becomes evil. This is just the way it is, don't try to get too cute and work up a contrived situation in order to prove you're smarter than everyone else in the room.
And for the record, I heartily agree with Uncle Jim's scenario of good v. good being more interesting. My point, and I make all my points based on military type thinking, is that once the conflict starts someone is forced to switch sides. They might not know it at the time but it will happen. Think of two opposing armies. They cannot both be on offense at the same time. If they start out that way it will quickly become apparent who has the upper hand and one side will switch to defense. If they both go on defense then you have no conflict. Therefore, you have to have offense and defense. In the same way, once a conflict starts someone has to be in the wrong. Otherwise they all sit around the campfire and sing Kumbaya."