As a man who has sucked his fair share of nipples (what, I was thirsty!) I can tell you that there is great range of women's nipple sensitivity and/or derived pleasure. Some truly enjoy it. Some pretend to. Some could take it or leave it, some don't like it much and some can't really tolerate it. Now, if your nipples hurt every time they are sucked, and the sucker is not the same aggressive boob, then you've got yourself a pair of nips that don't like it. There's no shame in that. Don't knock your knockers. They aren't "overally" sensitive, they are just in the high percentile of sensitivity.
Keep in mind though, if the pain persists AT ALL, or is very, very painful go see a doctor. Sometimes very senseitive nipples can mean more. Don't google freak out on it. But make an appointment and make sure everything's copacetic. In the mean time, tell your man to suck on something else. Your body is not blindly his for the taking; it's yours for the sharing.
Tell him to lick them instead. See if that works better for you.
But what do you do when you just can't stand having your nipples touched, licked, sucked, etc.? How do you tell someone that you don't like it? It always gets awkward for me.
Hey, I have been the receipent of such bad news. Disappointing yes, but since the nips in question were not attached to MY body I had to accept/respect that. I prefer the heads up beforehand rather than the loud offended yelp when I gently run my tounge over the nipple.