Because we live in an asinine culture that worships youth and considers it to be the pinnacle and be-all and end-all of human experience.
No, I'm not particularly liking having my metabolism slow down and putting on weight from just looking at food I used to eat with abandon. Not particularly liking having a bit less energy, or knowing I'm just going to have less from here. Yes, there are some things about my 20s and 30s (might as well say it, as I'll be 40 in less than six months) that I miss.
But, you know, whatever man. I've stayed alive this long. My mother didn't get that lucky. On the whole, while I might want my 30s back to live in a way that didn't get entirely swallowed by anxiety, depression, pouring out my heart and soul trying to be neurotypical, poisoning myself with psychotropic meds, and accruing buckets of regret, I'd rather go forward than go back.
Soon, it will be socially acceptable for me to do things my own way, speak my mind, and no longer give a damn what is considered acceptable or what anyone thinks (at least, as long as it's helpful and kind and maybe if I'm lucky wise). I think slower metabolism, lower energy, and the onset of arthritis are a fair price to pay.
Our youth-worshipping culture can kiss my ass.
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"Alas, our dried voices when we whisper together are quiet and meaningless, as wind in dry grass, or rats' feet over broken glass in our dry cellar." --TS Eliot, "The Hollow Men"