[ She's gonna be really embarrassed if Aerith ever meets Peter, with or without the mask. ]

Well, those who make strong bonds with the world and with a wide selection of others tend to stay longer, especially if those others have also made strong bonds. Though once your strongest bonds are gone, sometimes you will be, too. And then, sometimes you're me.

It gets a little less clear once you surpass a certain point. After two years or so, it's mostly luck of the draw.