Stalking is unwanted or obsessive attention by an individual or group toward another person. Stalking behaviors are related to harassment and intimidation and may include following the victim in person or monitoring them. The word stalking is used, with some differing meanings, in psychology and psychiatry and also in some legal jurisdictions as a term for a criminal offense.

Rejected stalkers pursue their victims in order to reverse, correct, or avenge a rejection (e.g. divorce, separation, termination).
Resentful stalkers pursue a vendetta because of a sense of grievance against the victims – motivated mainly by the desire to frighten and distress the victim.
Intimacy seekers seek to establish an intimate, loving relationship with their victim. Such stalkers often believe that the victim is a long-sought-after soul mate, and they were 'meant' to be together.
Incompetent suitors, despite poor social or courting skills, have a fixation, or in some cases, a sense of entitlement to an intimate relationship with those who have attracted their amorous interest. Their victims are most often already in a dating relationship with someone else.
Predatory stalkers spy on the victim in order to prepare and plan an attack – often sexual – on the victim.

https://en.m.wikipedia.org/wiki/Stalking