Why would this guy spy on me when I don't know him?

Sorry, but you sound kind of like a snob or something. For one thing, there is nothing wrong with someone overhearing you talking about something. You are in public and near the guy. If you don't want him to hear you, then go somewhere more private. As for him saying this kind of thing to others when you can hear him, he is simply using the info he's managed to pick up from overhearing you, and using it in conversation near you to show that he has something in common with you. I think it's clever, cute, and obviously he likes you. You should stop being such a snob about it.