Why do girls show interest and then avoid you?

I'm so sick and tired of this. A girl acts like she likes me (trying to talk to me/coming up with excuses to be near me/smiling and making eye contact) then we you show interest back they say, "I have a bf" Or "I'm not looking to date right now". Wtf?
Wtf
Wtf
Why do girls show interest and then avoid you?
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