1) You and your partner have a shared history so you can have conversations that make no sense to anyone else: "You remember that time when..." (perform odd hand gesture and make whistling noises) "Of course I do!" You know they know exactly what you're talking about.
2) Your partner remembers who you really are. You may not look like your younger self to anyone else, but to each other, you're the same.
3) Chances are you are not going to grow old alone.
4) Your partner knows what your weaknesses are and can jump in and give you a hand when you need it.
5) They know what your strengths are and they aren't threatened, they think it's cool that you are awesome.
6) Farting, belching, scratching, morning breath, it's okay.
7) You will never have to recount all the horror stories of your youth to another romantic partner. I mean, damn, that's tedious.
8) You know that arguments and angry feelings don't signal the end of anything.
9) Your partner has seen you at your worst and they're not afraid.
There's more. But I have to stop now. Kids and stuff.