Stalking is defined as any harassing or unwanted behaviors which one person perpetrates against another person. Stalking does not always rise to the level of criminality.
Stalking is a distinctive form of criminal activity composed of a series of actions that taken individually might constitute legal behavior.
One way to discover why stalkers stalk is to discover what the most common stalking behaviors are. The most common stalking behaviors include giving unwanted phone calls, flowers, or gifts; violating restraining orders; following, spying, or watching; and stealing private information (such as mail or video footage).
Obviously some stalking is done by people who have never actually met the people they are stalking; we hear about those kinds of things particularly in terms of celebrity stalking. Statistics reveal, however, that most victims know their stalkers. If that is true, stalkers do these things because they either want to continue a relationship that has been broken, for whatever reason, or because they want some kind of revenge for wrongs (perceived or real).
While there are some violent and/or delusional stalkers, most of them are men (about 70%) and women (about 30%) who are connected to the people they are stalking. Ex-spouses, co-workers, ex-lovers, or anyone else who feels possessive or obsessive about another person.
So, stalkers do what they do because they either want to stay connected to someone who does not want the same thing (or it would not be considered stalking) or because they want to seek revenge for wrongs they feel have been done to them. The delusional stalkers have mental problems which cause them to obsess on someone for a variety of reasons.