There’s stalking: following people home, writing endless letters, having a little shrine in the spare bedroom.
I’ve not done anything like that in decades, thankfully.
Then there are lower-grade facsimiles of stalking: not taking no for an answer, peeping, prying, poking around, staring, deciding that Someone is going to be one’s friend and making constant overtures despite lack of response or reciprocation, repeatedly contacting people who, by their failure to respond, are communicating very clearly indeed their unwillingness to be in communication, thinking about people who aren’t in the room.
I’ve had to learn to leave people alone, in practice, and in my mind.