They show you a side of theirs that the world will never see, just like how you are only truly yourself when you're with your partner.
You never imagined that life would become so simple, easy, fun, romantic, chaotic, silly, and sometimes childlike, all at once when you feel so deeply connected to someone. You and your partner become so comfortable in the relationship that you are never afraid to be truly yourself. And when nobody's watching, that's when you both are in your own little comfortable world, doing the most typical couple-like things, as Dina Odess depicts through her illustrations on Instagram. Take a look at them and you will surely relate.
...and your partner has no choice but to hear you sing it in the bedroom, in the shower, in the car, at the dining table, basically everywhere.
Partner: "Do you want some walnuts?"
Partner: (Opens a few extra ones because they know when your "no" means "yes")
...and sometimes forget that you're in public. But you know that with them, even doing laundry becomes fun.