Hi Girls. Why do they stare? Is it normal?

Happens to me aswell. I was in town for a few supplies a week ago, and I kept getting stared at. There was one girl in particular that constantly stared at me. We walked past eachother about three times (It wasn't a large shopping area) and she kept staring. She went out of her way just to stare at me. Say I was in a shop and she was standing just near outside the shop, she would move her head to see in through the door to stare at me. I stared back for a brief moment, but she turned her head in a very shy manner.
I think I'm attractive on rare occasions, but overall, I don't see myself as attractive. Although it may seem like I do at times, I really don't believe I do.

Girls complain about guys staring at them, but it happens to us fellow guys aswell. I don't know if it boosts my vanity or insecurity when it happens.