Same reason anyone has odds with anyone else really: Difference in life values. The Romulans can be a democracy that respects its' people without having the exact same values as the Feds, perhaps they see it as okay to go around settling on worlds with native populations and interfering with their development as long as they get a new world. Maybe they just feel they have a manifest destiny (which is pretty much canon...) and the Feds are an obstacle.
You can make up anything really, without making it so obviously a "Military Empires are inherently bad for everyone including themselves!" stuff.
Note, I am not advocating a military dictatorship as an ideal government. I'm just saying there's greater story potential for Trek in an antagonist who IS a democracy and not just another fascist Empire.