If you smile at a girl and she doesn't smile back what would you think, guys?

If I'm in a public place there will usually be at least one or two cute/ attractive guys that will catch my eye and smile at me and I tend to just seem to stare back at them and in my head I think "for god sakes woman SMILE!". This happens almost all the time but it's not like I don't want to smile its just my face physically won't move and I feel like I look like a massive bitch, but occasionally there will be a guy that is disarming and I smile back. So guys what would you think?
If you smile at a girl and she doesn't smile back what would you think, guys?
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