There is definitely a big difference between the heat in the southwest and the heat in the northeast. I lived for years in the Mid-Atlantic and New England, and when temps rise into the 90s, it is miserable. The humidity is horrible and the majority of people in New England do not have central air conditioning. Fortunately, most of the summer remains in the 70s and 80s.
We have been in Arizona for two years now and it is HOT in the summers, but 100+ temps here are much more tolerable than 100+ temps in New England. There is no humidity and, like you said, it does cool down at night. And every place has air conditioning. HOWEVER, the heat lasts A LOT longer here. No humidity, but the sun is relentless!
We are still adjusting to the southern Arizona summers.

It's easy to pick out the native Tucsonans. They're the ones out jogging and riding bikes in the heat.

Temps in the 100s is ho-hum to them.