Revenge body. Why do women transform when they get cheated on?

My sister-in-law got cheated on, she started going to the gym, lost a TON of weight and started taking care of herself. Then when she convinced him to come back she put on the weight again. Another woman I know, also got cheated on, she begged him to stay but he left her for the other woman and now she's going bananas. She lost 50 pounds, got a boob job, hair implants, fillers, and is now prepping for a lipo and bbl. I could go on and on... Have you heard of that happening? Why can't women just try and be as beautiful for themselves. Why does it take getting cheated on for them to invest in themselves? EDIT: @UNICORNSSTILL EXIST I totally get what your saying, but with my SIL why would she stop taking care of herself again once she got him back? Isn't this a toxic mentality, that you should only care for yourself to have a man?