Perhaps some people imagine, if they didn't die, they'd grow old; If imagine someone imagining being old and stuck with maximum walking speed of 2km/h seeing and hearing ability continuing to deteriorate. It's not hard to imagine they won't want to live forever in that state; There's a difference between being young forever and living forever.

Also if you were indeed young forever but everyone else wasn't, it's also not hard to imagine your life would be carefully scrutinised by everyone else for research; which not everyone might like.