Answer: Because they are creepy, trying to murder (sed person) a fanboy of the person they're stalking, or they have some sort of mental problem that makes them want to start stalking someone they know such as a family member, friend, past enemy, current enemy, or crush.
Explanation: This can be caused by all sorts of trauma, past experiences that greatly enhanced their lust for that person, and so on. They could just be a psychopath which ties into the point of my mental issue about the person stalking the victim and they are most likely trying to kill them.