Why do men flurt with you and are nice to you then when they find a girlfriend treat you like complete garbage?

this one shop keeper was so nice to me until recently when he found a girlfriend now he is constantly telling me off in the shop. seriously i was interested in him anyway because i knew he was kind of funny but that just show how important it is to know someone before you start liking them.

Why do men flurt with you and are nice to you then when they find a girlfriend treat you like complete garbage?
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