Perhaps he hopes to relieve tension in this way?
Going to the dentist can be a stressful time: the idea of having to lie in that chair, under that blinding light is no fun at all. Maybe that changes for this guy. He wants to find the funny side of this situation and decided to have a video made by his friend who accompanied him to the dentist’s office. The dentist is distracted, working on the computer, and this guy pulls out his huge cock as soon as he lies down in the chair. Is this a way of downplaying? Either way, this guy is an exhibitionist, he likes to show off his cock, and he’s done nothing but good.