Although I was masturbating from an early age, and my nipples would occasionally be painful under my school shirts on cold mornings as a teen, I didn't discover how wonderfully *sexually* sensitive they were until my early 20s.
It all came from an article in, I think, Forum magazine, back when it was a genuinely informative publication about sexuality, rather than just a tacky porn mag.
The article had a title along the lines of "Men - you can have sensitive nipples too!" and it gave advice on how to achieve that. I don't really remember the details, but the main thing I took away from it was that one should play with them whilst masturbating to help reinforce a sexual connection.
So the next time I was playing with myself I did precisely that. It just felt weird at first and added nothing, but by about the third time I tried it I could definitely feel a naughty tingly response, and I just went from there.
Before long my nipples became an essential part of my sexuality, and by the time I was in my 30s I could - and still can - be brought right to the brink of orgasm by nipple play alone. Rubbing, licking, sucking, pinching, pulling, twisting are all great... but NOT biting. No teeth!!
In fact I now couldn't be with anyone who didn't like playing with them. Luckily I've never found that to be the case, and indeed one lady I dated a few years ago was overjoyed, as her ex-partner of 22 years had always forbade her from touching his nips despite her wanting to, so she very much enjoyed going to town on mine!
I've tried various types of nipple clamp over the years, but in my opinion nothing beats soft-grip clothes pegs from the local hardware store or supermarket.
I've read several times over the years, indeed in this very thread, women saying that their nipples are hotwired to their clitoris. I'm similar, except that although there is SOME connection to my penis, there's far more of one to the lower third of my buttocks. Having my nipples played with makes my bottom tingle and squirm and crave being bent over and soundly caned for feeling so naughty... this also has the secondary effect on my cock that you'd expect, but it definitely affects my bottom first.
I guess that sort of nervous system wiring is one reason I've always been a spanko.