Originally Posted by vkn1
The reason men have nipples is plain and simply that they are of the same species as women, women need to have nipples, and the two sexes are differentiated not by fundamental major physical differences like the existence of nipples, but rather only in the form taken on by these features - by effects that occur as the result of hormones that orient the fundamental physical features in different directions. Breasts are the same between men and women except that female hormones have caused them to develop the capability of lactating (which men can do too if given female hormones).
What you state above is correct but the real reason is different. The embryo is in the first 6 weeks of pregnancy female. Between the 6-8 week of pregnancy the embryo gets a lot of male hormones (if it is intended to be a man) and this will lead to the birth of a boy. Because of this reason doctors can never tell the parents if it becomes a boy or girl if the embryo is younger than 8 weeks. An “heritage” of those first 6 weeks are the nipples, which are fairly useless for a man. This doesn’t only happen to humans but to (nearly) all mammals.