Because they are kept covered and get warm and sweaty. Feels good to air them out. Also, feels good to be able to just let them out and not feel like someone is hating on you. In private, with a friend (or friends), at a nude resort or beach, etc.
I feel sorry for people in very hot countries - I suppose wearing an undergarment in these areas doesn't help and you yearn for chance to get home and get naked - I don't have the guts to go to a nudist colony
You feel free, like if you are flying over the ocean with a fresh ocean breeze right through your parts... wonderful😭❤️ Cough cough* ehhh i mean the private parts have a lot of sensory nerves and they mostly activate happiness hormones such as serotonin, dopamine and other endorphins. This consequently leads to... yes exactly, happiness or even euphoria, which explains why it feels best when air goes through our private parts😜❤️hahaha