Can you explain what's the reason? You can say positive as well as negative reasons.
I think the most common reason for war is the urge to capture resources owned by another jurisdiction. For example, the USA frequently goes to war to secure access to energy resources.
amm.. None? There is no valid reason for you to take a life in order to gain your personal wants such as resources, power or jurisdiction. No one wins with war, you might think that once you annihilate all of your opponent you have won the war right? You have eliminated all that opposes your ideology. But what if one of your followers opposes you? will you eliminate them as well?
War is ignited by two opposing opinions or ideas. That's why leaders are chosen so they could decide for the majority's welfare not just a collateral damage because of personal agendas.