Why is wanting to look good frowned upon?

For both genders by the way, but I've had flack predominantly from body positivity people that I'm doing a disservice by posting gym videos with the tag body positivity.
Why is wanting to look good frowned upon?
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Not hating on plus sized people, if you're happy great but why on earth has going to the gym meant you're insecure or vain?
Why is wanting to look good frowned upon?
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