The Book Of Eli actress explained that their romance has been a success because they keep it out of the public eye.
"It's a very simple, pleasant, long relationship," she told Parade. "The reason we've been together for so long and the reason that we have such a healthy relationship is because we don't want it to be in the press. And we don't want the press to analyse what we do and how we do it."
However, Kunis added that the most important thing in a relationship is laughing.
"You have to be able to laugh with your partner," she said. "They have to be able to make you laugh, and if you can make them laugh too it's a plus. If you can't laugh with each other, it's going to be a very sad relationship.
"Also I think what makes a perfect couple is the differences between them, whether that be looks or personality. What's attractive to a female, more times than not, is the guy's personality. If he is able to make you smile or able to make you feel comfortable and safe, that is much more of an attractive feature than his actual superficial facade because that goes away very soon."
Kunis has previously said that marriage is not important to her.