I think there is a fine line between 'stalking' and stalking.
I think it's fairly common if you like someone that you will try to be where they are and try to learn more about them. In this sense it's a need to try to learn more about them to try to relate in a way that will have them in your life - for example when you're attracted to someone you learn where they hang out so you can 'accidently' bump into them then get chatting, or just to see them to make yourself feel good when they smile back.
I think it's hard to always see the boundaries, I'll I will openly admit I've maybe been in a bit of a grey area with people - putting in too much effort to be where they are and learning about them, based on obsession. I am not a stalker, just to make this clear.
I think when someone is a stalker there is an obvious line that has been crossed, their obsession will step-up a gear so that they're always around a person, so they not only know more about a person but start to use this information in an inappropriate way. But they probably don't see it because they've gradually slipped into this over time and can't see the difference between trying to be involved in someone's life, and actually forcing themselves into that persons life. The advantage is the same as the above, trying to be in their life, only when it gets to this point they actually ARE in the persons life, they're under the delusion that this is the natural progression of their 'relationship' with that person, and that they have thus achieved their goal of being in a relationship with this person they're stalking.
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Bloodheart
Good-looking girls break hearts, and goodhearted girls mend them.