"I've learned that sometimes love is not enough," Simpson, 33, tells Redbook in its February issue, on newsstands Jan. 14. "You can have a soul mate and be madly in love with that person, but it's not necessarily enough."
So what is the secret to a long-lasting relationship?
"I think you have to have personalities that mesh well. You have to be the yin to their yang. You also have to be open to figuring things out together and communicating even when it's the last thing you want to do," adds Simpson, who divorced first husband Nick Lachey in 2006.
Photographed for Redbook by Matt Jones
"I have to admit, it doesn't hurt that Eric is one of the hottest men I've ever seen and he looks amazing naked!" she jokes.
But on a much more serious note, Simpson told the magazine that a downside to her pregnancies with the couple's children – son Ace Knute, 6 months, and daughter Maxwell Drew, 18 months, was the media scrutiny over her weight.
"It's ridiculous and unfair. I never listen to it, no matter who the press talks about when they're pregnant," she says.
"I think any woman who is pregnant and creating a life is pretty much entitled to eat whatever she wants as long as she's healthy. I wasn't going to let the media take away from what was one of the happiest times in my life."