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.
War can be caused by invasion and war could mean death and destruction of properties. This can be a reason for people to support war. The preservation of lives and to defend your properties.
The preservation of lives is not a reason to support war, moreso it's a reason not to support it. Do you have an idea how many innocent lives are lost during war time? There aren't any winners there. There should be an alternative to resolve conflicts without wasting lives so there's basically no reason to support such barbaric acts