Why did the Roman government have large expenses?

The governments of ancient Rome had large military expenses. At it's peak, the Roman Empire stretched northeast from Rome to Scotland, east across North Africa, north to Germany and Scandanavia, and west to Turkey. Soldiers and sailors had to be paid, weapons, armor, and provisions supplied, forts and ships built, and roads and harbors maintained in order to defend the Empire from those who chose to attack and/or break away.