Why would your partner get insecure if you are becoming more attractive?

Why would your partner get insecure if you are becoming more attractive?
I don't get it.
In my last question (How would you feel if your partner got fit or more muscles for you? ) I had a lot of people telling me that me getting more fit and building up muscles is making my girlfriend insecure. How is that? She's very attractive and fit herself so I don't see why I should not try and be the same.
Any thought?
Why would your partner get insecure if you are becoming more attractive?
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