I grew up eating very bland Midwestern/Wisconsin food - lots of casseroles, canned veggies, tater tots, that kind of thing. I was not a black pepper fan, either. Starting in college, I started intentionally working at improving my heat tolerance so I could enjoy more foods. Taco Bell mild sauce was one of my starter items, beginning with a little bit and working my way up.
Two things stand out as really contributing to my efforts, though. One was when I moved out of the dorms and started cooking my own food, and didn’t want to live on hamburger and mac-n-cheese like my roommates. I also went vegetarian, and at that time, some of the easiest to find veg cookbooks that weren’t bland tofu and brown rice were ones that were full of recipes from around the world, especially Chinese food. The other thing was my first taste of Indian food - my then-boyfriend (now husband) took me to an Indian restaurant near campus. He was from the Chicago suburbs and had eaten Indian food before. The waitress was a definite “mom” type, who told us that she had them make our food a little milder than we’d ordered (I said mild, he said hot) because we were new to the restaurant and they made hotter food. Turns out this was wise - both of our mouths were burning but not quite intolerably so, and I was hooked on a new cuisine and new kind of ‘spicy’ that I hadn’t encountered before.
My efforts did work after not too much time - when my sister, mother, and aunt came to visit me at college and we went out to eat, both my sister and I picked a vegetarian burrito. I wolfed mine down happily, while it was too hot for my sister to eat more than a bite or two. Our aunt was a big Mexican food fan and finished it for her while she got something else to eat instead.
I prefer a heat level that causes mild sweating, but not at the expense of actually tasting the food. If I miss important subtleties in the flavor then the heat is not worth it. I grow serrano peppers at home every year and use them in place of the lower-Scoville jalapenos as I like the flavor better. I can wolf down a half jar or more of ‘medium’ heat commercial salsa in one sitting if left with it and a bag of tortilla chips. I also try to have a few hot sauces in my fridge, as well as salsa, and my two home-canning projects this year (so far) have been salsa and hot pickled peppers. (And chipotle peppers are awesome!)
The hottest food I’ve had recently was at Rick Bayless’ Frontera Grill restaurant here in Chicago. They normally put out two salsas (verde and regular), but my husband asked for some habanero salsa as well. He was scooping chips into it during dinner, obviously feeling the heat in the process, and I finally tried some - dipped in a chip and got maybe a quarter-sized puddle of it. Gulp. Oh my, that was hot. I had trouble speaking for about a minute, and drank a fair amount of water trying to get back to normal. It tasted really good, but I’ll have to work up to that.