When there is an economic expansion, demand seems to outpace supply, particularly for goods and services that take time and major capital to increase supply.
As a result, prices generally rise (or there is at least price pressure) and particularly for goods and services that cannot rapidly meet the increased demand such as housing in urban centers (relatively fixed supply), advanced education (takes time to expand/build new schools), but not cars because automotive plants can gear up pretty quickly.
Second, when there is an economic contraction, supply initially outpaces demand. However prices for most goods and services don't go down, and neither do wages.
Of course wages per se don't go down (but employers lay off workers so wage costs fall.)
Given that a recession is two quarters of negative growth we can see that if a shift of AD to the right is growth (actual) then negative growth is a shift to the left.
And yet prices of most goods and services do not fall.
Thus recessions don't exist!