اقتباس:why would men have nipples?  

Human beings, like all mammals, follow a similar developmental path regardless of their sex. In fact, the default sex of humans in early development is female. It's only the production of the male hormone testosterone that turns a fetus into a male.  

"Up until puberty, men have essentially as much mammary tissue as females," explains Dr. John Heddle, a biologist at York University in Toronto.  

That's when the female hormone estrogen leads to further mammary gland development and enlargement of the breast that will allow the production of milk. In males, the hormone androgen prevents this from happening.  

So, is it possible for men to lactate?  

Absolutely. If a man has a hormone imbalance (or if he is injected with enough estrogen) men can also further develop their mammary glands and produce milk – but probably not enough to nurse a newborn.  

Still wondering why men should have something they don't actually need? According to Heddle, don't go looking to evolution for an answer. Evolution, he says, tends to favour only those characteristics that we actually need. As for things that we don't need, evolution doesn't really care one way or the other (unless it requires additional energy to maintain or threatens us in some other way).  

So the male nipple is just one of those things that has ornamental value and not much of anything else. Kind of like cufflinks. But that's another story.  
Chang.