Most raw vegetables, like our bodies, are very perishable. When raw foods are exposed to temperatures above 118 degrees, they start to rapidly break down, just as our bodies would if we had a fever that high. One of the constituents of food that can break down is enzymes. Enzymes help us digest our food. Enzymes are proteins and they have a very specific 3-dimensional structure in space. Once they are heated over 118 degrees, their structure changes.
Eating enzyme-deficient foods place a burden on your pancreas and other organs and it overworks them. This will eventually exhaust the organs (just like putting the wrong fuel into a car).