Why do women flirt more with everyone else right before launching a relationship?

I have a friend who three weeks ago suddenly seemed like she was into me. Had been texting me more often than usual, ran into her at a party, seemed like she was kinda following me around, flashed me a <3 with her hands while we were playfighting, two days later she asked to hang out (which is the first time she's ever asked to hang out alone together, and she's turned down my previous invitations to parties).

Then she cancelled, and tonight I met the new boyfriend. Not the first time that basically this scenario has happened in my life. What gives?

Why do women flirt more with everyone else right before launching a relationship?
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