There's a common belief out there that male nipples aren't as sensitive as female nipples. I'm not really sure where this started or how this came into being but I've met many people who have held this belief.
I seem to remember reading a little article in a fairly serious publication about 12 years ago that men actually have a higher concentration of nerve endings in the nipples than women since women are designed to breast feed and their nipples are meant to take a bit more "abuse" without experiencing. It could very well be wrong information. I tried to do a bit of research on this subject but wasn't able to dig up any scientific evidence in support of either gender having a greater concentration of nerve endings, so for the purpose of this post I will be working under the assumption that male and female nipples can be equally sensitive.
I use the phrase "can be" since many men seem to be a lot less nipple sensitive than women. Also about 12 years ago (during my piercing phase) I read on a non-scientific website that men's nipples could be as sensitive (if not more sensitive) than women's, but since men have tougher/thicker skin (and often excess chest hair), it can take some work to build the same kind of sensitivity. Basically they said that male nipples had to be "activated."
As of that time I didn't have very sensitive nipples and I explored some of the methods listed on the site for making them more sensitive. Some of the methods described were to gently rub the rough skin around the nipples until they are erect and then holding them firmly and gently twisting them from side to side, rubbing them back and forth/up and down through a shirt/sheet/layer of fabric, etc. It took a couple of months of doing this while I fell asleep at night but very soon after I found nipple stimulation causing intense arousal and after a while my nipples became incredibly sensitive and able to trigger erections if they were teased.
Since then I have become a fairly firm believer in the idea of awakening male nipples through regular contact and nipple stimulation has become one of the more pleasurable aspects of erotic play.
If you are a sub that doesn't have very sensitive nipples (or a Domme whose sub doesn't have very sensitive nipples) this may be a tactic you could try in order to increase arousal and sexual intensity.