Boredom. One society has become so advanced that everyone is basically immortal, with instantaneous memory backup and rapid cloning. Material losses are irrelevant and even death is only a minor inconvenience. After a few thousand years of this, people got so desensitized to this that to them war is just a break from the routine, regardless of whether this is true for the other side.
Or maybe a young generation is fed up by their stale/codified/ritualistic society and war is seen as a return to a nobler/purer/truest past, even to the point of rejecting immortality technology and looking for true death in battle.
Or maybe a young generation is fed up by their stale/codified/ritualistic society and war is seen as a return to a nobler/purer/truest past, even to the point of rejecting immortality technology and looking for true death in battle.