The 43-year-old actress discussed her relationship with the 37-year-old musician on Thursday's episode of The Ellen DeGeneres Show, citing her new self-awareness as the best and worst part of being married.
"Someone asked me, 'What's your favorite thing about being married?' And I was like, 'Well, I just learned so much about myself. I didn't know these things,'" Diaz recalled. "It's like, I would have never known them if I didn't meet my husband, who helped me.'"
"[At the same time,] I learned all these things about myself that I wish I never learned about myself and I wouldn't have known it if I didn't get married," she admitted. "So it's like this double-edged sword. But I'm just so grateful for it because I couldn't do it without my husband."
Diaz and Madden met through Madden's brother, Joel Madden, and Joel's wife, Nicole Richie. The couple were engaged less than two months before they married in January 2015, and Diaz couldn't be happier.
"Happiness actually has shown that when we're happy we live longer. When we accept that we're aging, we live longer," the star said. "Our relationships are very, very important. Any kind of relationship, it doesn't just have to be a marriage."
"It has to be, basically think of it as a purpose for living," she explained. "Even if you're just engaged in something that you really think is important. But I do think that marriage, my experience with it, when you can grow with someone, it's very important."
Diaz previously dated Justin Timberlake and Alex Rodriguez, but recently told Radio Andy her past is immaterial with Madden in her life. The actress said her relationship with Madden taught her what "real love" is, causing everything else to "dissolve."
"Nothing matters now that I have my husband," she declared. "I don't even remember any of that. That's why I know he's my husband, is because no one compares. Everything else just kind of like washes and slips away."
Diaz appeared on Ellen to promote her new book, The Longevity Book: The Science of Aging, the Biology of Strength, and the Privilege of Time, which was released April 5.