Why do humans have two nipples?

Actually, the reason men have nipples is because both males and females must carry enough DNA to reproduce humans of both genders. So both males and females have the stem cells needed for growing breasts. But since only females have ovaries, generally only maturing and adult females produce enough estrogen to cause their breasts to grow. Males can on occasion develop fully-mature breasts, and that's usually due to a hormone imbalance or due to medication.