I found that full of milk my breasts have almost no sensitivity. With the milk dried up, though, they're very sensitive. In addition, the way a baby latches and sucks can almost be painful, whereas the way my partner plays is almost always pleasurable. My guess is, like in almost everything it differs from woman to woman.

I've also had male partners that love their own nipples touched and sucked on, so it is not a woman only experience, I think men just don't feel as comfortable talking about their pleasure from it.