This article is very good. Really, you should read the whole thing. It was in the New York Times a few weeks ago: "The Happy Marriage Is the 'Me' Marriage."
Essentially, it explains that people who are happiest in their marriages find that their partners "make their lives more interesting" and each one finds they want to be better because of their spouse and relationship.
"People... want partners who make their lives more interesting... The best marriages are those that bring satisfaction to the individual... and... research shows that the more self-expansion (accumulating knowledge and experiences) people experience from their partner, the more committed and satisfied they are in the relationship."
"If you're seeking self-growth and obtain it from your partner, then that puts your partner in a pretty important position."
"Additional research suggests that spouses eventually adopt the traits of the other - and become slower to distinguish differences between them, or slower to remember which skills belong to which spouse... It's not that these couples lost themselves in the marriage; instead, they grew in it. Activities, traits and behaviors that had not been part of their identity before the relationship were now an essential part of how they experience life."
"People have a fundamental motivation to improve the self and add to who they are as a person... if your partner is helping you become a better person, you become happier and more satisfied in the relationship."
Luckily, I think my husband and I find each other pretty interesting (though he may sometimes be pretending when I am spewing off details about daily potty-training experiences). We think highly of each other and, I think, seek validation from each other. This is really helpful because it makes it really easy to get over arguments quickly. We can't stay mad for long because we need to share with each other things we've learned and experienced every day.
And, I like that. a lot.