A quick little search found this - which is an evolutional explanation:
Male nipples aren't exactly a genetic glitch: they are evidence of our developmental clock. In the early stages of life from conception until about 14 weeks, all human fetuses look the same, regardless of gender. At the tender age of 14 weeks post-fertilization , genetically-male fetuses begin to produce male hormones including testosterone. These hormones turn the androgynous fetus into a bouncing baby boy.
Here's where the developmental clock comes in. By 14 weeks, when the hormones turn on, the nipples have already formed. So, while our male fetus goes on to become a baby boy, he keeps his nipples, reminding all of us that people, male and female, started off the same way.
In most men, the nipples really don't change after this point, but some men can develop a condition called gynecomastia. In gynecomastia, the fatty tissue around the nipple develops and eventually appears similar to a female breast. This can occur whenever the testosterone level is lowered by medications, such as those that treat prostate cancer, and by natural hormonal changes due to obesity, adolescence or aging.
Luckily, most of us don't worry too much about male nipples, so men never have to worry about finding swimtrunks and a bikini top that fit.