The point about humidity has already been made. I was astonished at how comfortable 115 degrees was in Phoenix. I went on a hike, no problem... Except that I carried a huge water bottle and the bottle ran out in an hour, so there's that.

In humid places, however, sweating is almost useless and you can suffer in mere 90F heat.

The other important point is that somehow folks in Singapore manage to survive, but in Singapore there's never any winter. I once heard a physiologist remark that humans can adjust to a wide range of temperatures, but it takes a few days for the body to reprogram itself for freezing ambient temperatures after spending months in roasting ambient temperatures. That's why the first few days of cold weather in the fall and winter feel really cold and everyone dons parkas, but when spring rolls around the exact same weather feels really warm and everyone puts on their shorts.

So maybe if I set out to live without A/C I'd get more used to it after a while. But the first few days are a killer, and at some point one just gets sick of being listless and warm and starts subconsciously looking for excuses to visit nice air-conditioned buildings.