I live in a pretty hot climate, but it’s only really unbearable at times due to shoddy building and bad urban planning. Even then, summer can be difficult.
I can’t imagine what it’s like on the equator, especially in dense urban centres. What’s Mumbai, Bangkok, or Singapore like at the height of the wet season?! How do millions of people function day to day?
Mental conditioning and altered behavior.
If you’re not used to it the heat can feel oppressive. Some kind of lizard-brain response telling you that the heat is a threat that must be mitigated immediately. Once you get used to it you realise that it’s just mild discomfort and that it’s not harmful (except for sun burn, heat stroke, et cetera).
You also alter your behavior in simple ways that may not occur to you if you’re not used to a hot climate. Like… make mid-day the least active part of your day. You’re not going to just duck down to the shops for a bite at 1pm.