They fought for the same reasons that people fight in real life, because they believed that the other side was inferior to them, and they wanted to let them know by killing them. Just like present day wars. That's probably why Mystics and Reptites hate humans, because they at one point in time thought that they were superior, and after Lavos crashed, this wasn't the case, humans became dominant, and eventually stole land from Mystics (cathedrals) and it ticked 'em off, then they got their asses kick in the Mystic war. So basically they just wanted to blow off some steam, and some of them wanted to be killed by a luminaire or antipode 3. Nothing wrong with that, right?