A friend of mine once asked their partner during a breakup, 'What do you do to make my life better?' They couldn't think of a thing. It's something I keep in mind now.
We're awful at explaining to other people why we're happy in our marriage - it always comes off as having 'settled' but that's not it at all. It was more like we met, hung out, got together and then it was as if we'd been together all along, like they'd always been there.