I started shaving my underarms almost 20 years ago. My whole life I've been one of those people who is always overheating, so I regularly wax my hairy back too, and my buttcrack, and as soon as the weather outside starts warming up and regularly going above freezing, I shave my head weekly until the following winter. It all helps me to regulate my body temperature.
Plus, I kind of like it aesthetically. I was never a fan of armpit hair and I feel a lot cleaner without, same with the butt hair lol
My pits were the first thing I started trimming, long before everywhere else (more than 20 years ago). It was entirely personal preference - I just never liked the hair.