Why would you watch and eavesdrop on someones conversations?

Assuming you don't do that with everyone because your nosy. I've noticed people keeping tabs on me, listenting to my conversatoins in person and over the phone and trying to spy on me by watching me or "accidentally" bumping into me. I think it might be because I'm quiet and reclusive but on the other hand not every introverted person I know is being monitored like I am.

Why would you watch and eavesdrop on someones conversations?
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