Why don't girls give compliments?

ILuvBoost240sx
I've noticed that a lot of girls never give back compliments or tell me I'm hot, when clearly they have a physical attraction. This girl at my gym is constantly checking me out and gives seducing looks when we lock eyes. Finally I get her number and I tell her how stunning she is and all she says is thanks... This happens all the time. Often times with girls who are the ones that initiate contact in the first place! What gives?
Why don't girls give compliments?
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