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.
Just think of it as a game that when everything is so unorganized and there is no more order anymore, a reset is necessary. Some individual thinks that war is the best solution for everything to go in order once again. Although, I do not totally support them because of the violence it may bring, but we have to admit that they have a point.