Most men part their hair on the left side because that is the side on which their hair naturally parts. All people have at least one natural part in their hair, and when styling, it is generally easier to incorporate the natural part rather than make a new one.
The location of a man's natural part is determined by the cowlick, or spiral of hair, at the back of his head. If the cowlick rotates clockwise, the part naturally falls on the left side of the head, and if it rotates counterclockwise, it falls on the right. Some men have two cowlicks, which means their hair naturally parts on either side.