Deflation implies that the medium of exchange function of money has malfunctioned. You can't use money both as a medium of exchange and a store of value at the same time. It just doesn't work.

Inflation ruins the store of value function of money, but a little inflation isn't a bad tradeoff if the medium of exchange function of money keeps working.

The reason for this bias is that our society defines itself through a high degree of the division of labor. If money stops flowing, the division of labor stops and without the division of labor we are going straight back to the stone age (hyperbole). This is why economic recessions and depressions are feared more than economic booms.