Something has been puzzling me for a while.
Why does Word want to start a Dragon Human War?

Well, the reason mentioned is that Word thinks that humans are abusing dragons, so dragons should rule. Only that is if he rules them. See how it plays so nicely into his hands ;)
That's were the wraith gear comes in. He controls the dragons with it, and as soon as it is more advanced, he will be able to control almost all dragons in Dragon-City.

The best refence I can give you is the conversation between Moordryd and Lance in The Wraith Booster in that and a few other spots I don't remember off the top of my head, the general theory emerges that Word believes that the dragons are more fit to rule and that the current order with humans at the top is wrong.

He is partly correct, it's not right for humans to rule and subjugate dragons as they do, but he is misguided as to the true path. He believes that he can control the minds and spirits of the victorious dragons after the war, but the true path to peace lies in the cooperation between dragons and humans. (That part is seen most in episodes dealing with the Prophets.)

I hope that helped and if it didn't, don't worry. This is the driving message of the show in some respects. I'm sure that I'm not the only one you'll hear from.