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Plenty of people want to improve their appearance as a way to get their ex to want them back/“show them what they lost”. Of course what you described is the healthier way to go about it.
Spite is a powerful motivator. >

And cocaine’s a hell of a drug. >

I don’t think glow ups are meant as an attempt to get someone back. It’s usually a way to mark a fresh new start and to serve as a confidence boost, which is usually needed after getting your heart broken.