"Reasons long forgotten" Is always fun, and gives the option of a side road/goal for the characters/RPers. 
But if you wanna go with something more "Known". . . . . There are three gods, but two of them have been fighting since the beginning of time, and those who worship them/were born of them mimic their gods' actions down on earth. The only reason for the peace was because a strong enough groups of non devouts on both sides managed to take power and forced the more religious fanatics to stop killing each other for what they saw as such a foolish reason?
Both sides are part of a grand experiment set forth eons ago by some higher powers and have always been born with the innate need to fight one another. However as part of the experiment a generation was born without that innate need and when they came of age they sought peace because they did not feel, even with how they were raised to hate the other side, that all the killing was worth the price. However, now the children are once again being born with the need to fight, and the higher powers want to see if the peaceful generation will be able to keep the peace or if they will be overthrown by the older and younger generations seeking an all out war?
For generations the Nightwalker(Or Daywalkers) were used as slaves . . .Need I say more? :P

I think that's all I can come up with one the spot . . . .Well, sure I could do more, but I wanna see what you think, and what ideas you might have :P OH! And did I get all the pertinent information down up there, or did I miss something?