Stalking. This word can be defined as the unhealthy habit of following one person obsessively and without their permission. An element of trespassing is usually involved. Due to their mental instability, stalkers are very dangerous. It should be noted that stalking someone is a crime.
One of the most notable stalkers in recent fiction is Edward Cullen, a vampire who develops an obsession with a teenage girl named Bella Swan. He goes so far as to sneak into her bedroom and watch her sleep. Oddly, when he reveals this fact to her, she is delighted. This whole ugly mess is portrayed by the author as a positive thing, strangely enough. As a result, the more unhinged fans of the Twilight novels (the books featuring Edward Cullen) deeply desire to be stalked by a sparkly vampire (or, failing that, their boyfriend).
On the other hand, SC2's Stalkers are awesome, especially when paired with Zeratul, see of Doom Whispers of Doom.