Why do guys get insecure and act dumb when they are in relationship?

Yesterday I was at club with my friend and one couple sit next to our table. I had few eye contact with her n smiled. I went on dance floor after few minutes she went to washroom I thought she given me hint lol. I follow her n I said hi n you look so beautiful she smiled and her boyfriend caught us talking. My god the way he given her look so angry n they left club. 😂
Why do guys get insecure and act dumb when they are in relationship?
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