@The Pentacle Queen,
I know it might sound weird, but some of it comes down to politeness. Not in the same of being overly formal and holding someone at arm's length, though. Rather, it's more like thanking RP when he makes me a salad, or him asking me to please water the plants or whatever.
We don't take each other for granted. We don't turn it into nagging. But we both pitch in because it's truly appreciated by the other person.
Another piece is - chai2 had said this really well - e. g. know which hill you want to die on. That is, don't sweat the small stuff and don't get into small arguments because they are just not worth it. This does not mean just sitting there and taking it until you explode. It also doesn't mean that your own feelings are immaterial. It's more that you should take a stand about important things, such as where to live, how to care for an elderly parent, how to keep your financial house in order, etc. But don't dig your heels in about where to go to dinner unless you never, ever get your way. Don't go to extremes just because someone bought the wrong coffee creamer.