It’s all about the arrectores pilorum. What, you say, are arrectores pilorum? These tiny little hair erector muscles that contract and raise the hair follicles above the skin. These are goose bumps or goose flesh or chicken skin. What causes them? They start with a stimulus such as fear, cold, or the sight of yourself in the mirror after a night of vodka-induced debauchery. This causes the sympathetic nervous system to become activated. The sympathetic nervous system is responsible for the body’s “fight or flight” response. This sends a message to the skin and activates those little muscles.

From: Why Do Men Have Nipples? By: Mark Leyner and Billy Goldberg, M.D.