Why do men suddenly start flirting with a woman they never seemed to care about/be attracted to?

I noticed a few times that men (fellow students, coworkers etc.) who have known me for months and did not seem to be interested in me started to chat me up, be super friendly and even flirty. I never encouraged anyone to flirt with me and I don't see how men would benefit from making me feel desired or accepted. Do you have any idea why they might be doing this? I haven't changed my appearance at all.

Why do men suddenly start flirting with a woman they never seemed to care about/be attracted to?
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