Why do people get jealous over their good looking partner?

I never considered myself hot or very good looking , I have flaws that I don’t like about myself , I don’t think I am ugly , but I don’t think I am this hot ass guy , I just learned to accept myself for who I am , some girls’ I have been with would get jealous by thinking all these other girls’ want me and thinking I am going to leave her for someone else? Just because I talked to another girl it doesn’t mean I want to fuck her , I am just being polite , and yes at times I will see a beautiful girl and be like Damn she is fine but I keep that to myself to not be disrespectful. I realized girls’ that are jealous will tell me about another hot guy , or not keep it to herself , like it’s going to make me mad or upset and jealous that she finds another guy attractive, I do find it disrespectful , because I don’t do that to her. So why do people do this to each other? if they are choosing you the same way you are choosing them? It actually surprises me that she feels that way about me , like she is that worried I am going to cheat on her. I honestly
assume she is going to cheat on me, by the way she talks about other guys’ to me , so why do this to someone you chose to be in a relationship with? Why make your partner feel this way if you truly loved and cared about them?
Why do people get jealous over their good looking partner?
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