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Back then the services the state provided were less varied then nowadays. No healthcare, education, or public services. They usually offered a justice system and protection (maintaining an army). Since those who did the protection were those who owned the land the commoners tilled, they did the administration (collected taxes), and served justice, it was inevitable that the military become the ruling strata of society.
Now we could say for the same reason it was basically a protection racket - sure, why not - but their origins can be traced back to the role of protecting the Roman border (see dux and rex). So theoretically they were against foreign enemy.
Besides there was a contract between serfs and the lords - unlike the helots and spartans - and commoners only reached for violence when the nobles overstepped the boundaries arbitrarily. And ofc this is more nuanced question small landholders, belonging to the military "elite" were often found themselves on the side of the rebelling peasants since higher ups in the chain of vassals tried to force them down into servitude - again stepping over their boundaries.