It's all chemical. Some things, like caffeol (which gives coffee its falvor), can only be sensed by taste receptors in your mouth when they're volatile (aka, evaporating, aka hot). Similarly, fat, which is something your body (thus your tongue) loves, are dispersed by heat. That's why chilled cheese is pointless. What you're experiencing in the cold pizza is the reintegration of flavors separated by heating. The same phenomenon often manifests in cooking with wood. When the meat (or whatever) first comes off the fire you won;t taste the smoke very much at all. Let the smoke and meat condense together (which involves changes in the molecular level as temperature cools often producing new bonds that couldn't exist before and will survive reheating)) in the fridge and the next morning you'll taste a very strong smoke flavor.
next day you're hungry for it again and your mind and body compare it to the last slice you had the day before when you overate and were uncomfortable at the end.