So this is my advice*, on having a happy marriage, after 13 years of more happiness than not (though to be fair, some years might fit in the not category, but not many... less than a handful really... but I digress):
Say, "I love you."
Say, "I love you, too."
Say, "Thank you!"
Say, "You're welcome!"
Ask, "Did you remember to grind the coffee before bed?"
Say, "Oh, thank God. And you, too, of course. I love you. Good night."
Say, "I don't know how."
Say, "I know. It's ok. We'll figure this out."
Say, "I'm sorry."
Say, "I forgive you."
Of course, these are only the things we say, but it's the not quitting part that helps us mean it. It's what makes us get up in the morning and before we even think to brush teeth, reach over for a quick kiss and a good morning and how did you sleep? It's what binds us during the day with the don't forget and the we're out of milk! It's what closes the book, turns off the lamp, and rolls over whispering good night and i love you.
It's the not quitting that makes it stick. It means we're in it together.
And that makes us happy.
*Not a scientific study; based solely on a random sampling of two midwesterners trying to make a go of it, keep their wits about them, and still love God and each other. Results may vary. Consult your partner.