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.
Nations usually fight over resources and sometimes ideologies. But since war is an expensive effort (in terms of human life and economically), it is usually done to gain an upper hand on the other nation's resources (human capital, technology, natural resources).
Most of the wars currently active are the result of jockeying for influence by groups on resources. Some large nations also use it to keep smaller nations at bay and held them hostage to the larger nations whims.