How do you deal with a stalker?

First off, stalking is a crime. While it is different from other forms of crime as no specific adverse action defines it, stalking is the persistent unwanted behavior by someone towards you- the victim. The unwanted behaviour can include following you, collecting information about you, threats, and attempts to repeatedly contact you in ways you fear. Stalking makes you feel nervous and susceptible to any form of attack. Since the resultant goal of stalking is to make you feel vulnerable, what motivates stalkers to want to make you vulnerable? Why do they want to stalk you?