You'll get fat, or I'll get fat, and whoever doesn't will be a little embarrassed to be seen with the person who did, and then resentment will creep in and suddenly sex will suck, and then each of us will retreat to the insipid- like romance novels or a recliner and sports. It happens I've seen it.
You'll get religious, or I'll get religious- pious, zealous even- and want to start handing out tracts to people at the grocery store or going door-to-door on Saturdays and you'd want me (or I'd want you) to come with. That happens, too. I've seen it.
Or what if I turn out to be a bad parent and I- I don't know- I hit the kid? Or maybe I don't hit them, but I guilt or intimidate them? Even worse, what if you turn out to be a bad parent who is so confident that they're a good parent that they can't even see what kind of a demon spawn they're raising? What if you're indulgent and my kid ends up being an entitled brat? I've definitely seen that happen. Kids aren't like dogs. You don't get a do-over every fifteen years.
Maybe we get ourselves into a terrible mortgage on a property that can't be given away, or we put all our money into the stock market and it tanks- or, hell, we only make safe investments and never actually make enough to retire? Maybe I get cancer or you have- I don't know- Huntington's Disease; maybe you have terrible taste in decor; maybe I'm not as interesting to you as I am right now and you get bored and you leave with someone else; or you become like your mother and pissed off at everything and everyone and me; or I become like my mother, emotionally catatonic.
It happens. I've seen it.
Jesus- what if you become a Republican?!