It's the same principle of rooting against things. Like I don't give too much of a crap about NFL but I'll always root against the Pats. I don't watch MLB but I'll always root against the Yankees. People get motivated by spite or by wanting the fight much easier than wanting the feel good thing. It's why most wars are always rooted in dividing people into sides.
All wars are economic wars. Divisions are created to ramp up the serfs and get them motivated. Even the Indian wars before 1492 were economic - my people vs your people for access to resources.
It's at least honest when the warriors know what they're fighting for, something American GIs have been lied to about for a long, long time.