Gluten.

As in, I tremble with excitement just to rediscover that, after all, I'm still sensitive to gluten and in a few hours I'm going to need a bathroom. The best part of pizza is everything before that first bite and the crushing realization that I should've gone for a steak instead.

Most likely a lack of seretonin response. I'm ignoring the "pizza mouth" aspect, since that is your pain nerve receptors, but the "yes, this is good" neurotransmitter is Serotonin.

Usually oil in case of a sad, wet pizza...