From a biology view, maybe he likes the face the most because of bloodflow. Your face has more capillaries, and they are closer to the surface than most of the rest of your body. This is why small cuts on your face bleed so profusely. It also has a large surface area, and it may be acting as a 'heat mat.' As your level of enjoyment goes up, your bloodflow will increase causing the area to be even hotter.
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