Money, Millander. These people were soldiers as profession. In a few instances it was patriotism, like France 1793 (though still technically conscripts), Netherlands 1813, Spain and Germany 1813, but when you exclude the liberation wars, it's really just money.
Patriotic volunteers often formed their own units (the famous 'Volunteer Jagers) that they'd disband after the war had ended. Sometimes they were simply thrown into a regular battalion and became regular soldiers against their wills. Such a case happened with a company of volunteers from the town of Goes, in Zeeland, was added to the 2nd Batallion of the Line in 1814. Nearly the entire company deserted after they realized they were supposed to do 6 years service.