I'm glad you had fun with it :)

I never cease to be amazed at the way we can shift our perceptions like that.

Nipples are definitely high on the touch scale. A little lower regarding motion, but very pressure sensitivity, and only slightly than pressure with temperature. That's pulling from memory, but I'm fairly confident about my memory on the subject because it hasn't been long since I had to refresh on things.

As to why someone might or might not enjoy nipple play, that's complex. All of our perception via skin senses is a mix of the various nerve ending types, the thickness of the skin at any given spot, the brain's filters, psychological filters/associations, and mind frame in the moment.

Even if you ignore sexual arousal and sexual intent, we tend to think of nipples as something "special" in comparison to, say, the knee. So our minds set us up to some degree or another to process the sensations at the nipple in a fairly unique way. Since they're "bigger" than their actual size, everything from them is going to take up more room in the brain, the same way lips and such do.

That, btw, is about how much of the brain is dedicated to processing the signals from an area. I can't find one right now, but there's images of what our bodies would look like if they were sized in proportion to how much the brain devotes to the area. I'm running on empty right now, but I'll try to find one once I've had some sleep.

Back to the nipples though. Because of the job they do, high sensitivity is necessary. Remember, even the nipples on men are still the same basic equipment, so they follow the same resource devotion as women's do. They're all evolved with baby feeding as being a survival trait.

But the system isn't perfect. Sometimes, some aspect of the link between the nipples and the brain skew too far into sensitivity, and you run across the folks where just having soft fabric rub them can be outright painful. But the exact reason can vary based on any of the factors I mentioned earlier. It can be an unusually high proportion of pain receptors, it could be the brain filtering the signals weird, it could even be psychological rather than neurological or anatomical. But it comes down to the signals being individualized.

As an example, I deal with chronic pain. I've learned how to ignore some sensations that were enough to have me contemplating suicide when it all started. So, for me, it takes a higher level of intensity for my nipples to be perceived as painful, even when someone is practically chewing on them (or literally is, the lady I dated before I met my wife was intense lol). But, back before the chronic pain stuff, I had a much lower threshold where pulling and biting would become unpleasant. Learning how to compartmentalize pain in general, which is a combination of meditative and psychological practice, means that even though the signals of my nipple being bitten is exactly the same as before, the way my brain prioritizes and filters those signals changed.

I wish there was a simpler, more direct answer than it being a dozen individual factors, but that's what it is.

As far as why the perception is weird compared to other parts of the body, it is the nerve density and the thinness of the tissue of the areolae and nipples. They're set up so that feeding babies isn't overwhelming (as a baseline, because that can be way overwhelming for some people), but there's acute sensitivity for the process of feeding. They're also linked into the same involuntary nervous system that governs arousal (and orgasm!), so we tend to place a different weight on them when it comes to the brain and the mind. Nipples and areolae really are pretty unique compared to the majority of our skin surface. The lips are the most similar iirc, with parts of the genitals being close as well.

I'd have to go digging to find the rough proportions for the various sections of the body because it wasn't a factor in what I was looking for when I first ran across the subject as a whole. But every section does have a different proportion of the various nerve ending types, and different densities of them. But they also link to other sections of the nervous system differently, which means the link to the brain for a given section is going to vary wildly across the body.

We are marvels of evolution. Our reduced hair presence gives us a lot of extra sensory data, and we've got brains matched to be able to process the millions of signals from all of that every split second. With nothing but the little nerve endings connected to the short and thin (relative to other mammals), we can detect a breeze so faint as to not visibly move hairs. How fucking cool is that? We can pick up differences in temperature down to a few degrees. We can accurately detect pressure down to about a half PSI on our fingertips, sometimes even less.

A lot of what we learn about how we compare to other animals glosses over exactly how sensitive our skin can be. And, more importantly, how powerful our brains are to be able to process all of it.

The nipples are a perfect example of that. Did you know that some people can read Braille with their nipples? No bullshit, I used to date a blind lady that would do it as a party trick. She said she knew a guy that could do it with the tip of his penis too, though she may have been trolling me. Which is way tangential to what you asked, but I think it illustrates exactly how unique the configuration of the nipples is compared to other parts of the body.