This all comes down to "MV=PQ", a description of the relation between money supply and real economic activity. Normally as the real economy expands (+Q) the amount of money is increased to keep it growing (+M). Without that either the quantity of transactions falls (-V) or the price level falls (deflation, -P).

Alternatively, think about what price stability really means. If a loaf of bread costs $1, an hour of work $10, a barrel of oil $50, and a house $500k today ... how do you really ensure that they cost exactly that in 50 years when you come to retire? You can't. In a real economy there are real reasons for shifts in the price level.