Speaking in a language of inside jokes that only the two of you understand. Bonus points for weird voices that you only use with each other.
Falling into a nightly routine without even realizing it.
Doing things for each other that newer couples wouldn't dream of — plucking his stubborn unibrow hair, picking up her favorite tampons from the store, and so on.
Getting excited to have the house/bed to yourself (but then not being able to sleep the same without them next to you).