Your finger has a slight electrical charge, and the capacitive screen measures the change in the screen's electrical field and computes the touch point based upon that. A regular, plastic-tipped stylus has no such charge, rendering it useless on this type of screen. There are precision styli available for capacitive screens. If you want to do sketch work on the screen, purchase one of them.
Keep in mind however, that with the exception of the Galaxy Note 3 and 4, possibly a few others, capacitive screens are not as sensitive as a Wacom drawing tablet. The Note 3 and 4 both have a digitizer manufactured by Wacom.