A question about rumors and slanderers?

Why would people engage in slandering other people? What are the motives? A part from casual gossip and small talk, I feel that people go overboard with the rumor-mongerers who try to find every detail about your family relationships, friendships, medical record, personal interests, and seek to exert control over your life by interfering with yoru personal space? Is this normal? Or is it a characteristic of stalkers?