Ladies, what's the point of smiling and staring at a guy if you're taken or not interested?

The last couple of months have been very weird to me, as I've seen these two different girls in my neighborhood. Every time I see one of them they stare at me and give me that 'approach me' look, and sometimes they even smile at me. I'm the kind of guy who doesn't approach girls until I'm certain they're into me. Eventually, I did approach both of them, one of them told me she had a boyfriend and the other gave me her number but she didn't even reply to my texts... Yes, I'm good looking and well built but don't stare and smile at me every time you see me and expect me to think that you're not into me. I really don't get it.
Ladies, what's the point of smiling and staring at a guy if you're taken or not interested?
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