The former "Alias" star said that while she can't specifically identify the key to their successful marriage, she knows their positive attitudes towards others are not a hindrance, People magazine reported Friday.
"I don't know what makes (the relationship) work. But it does," Garner said. "One thing that makes it not not work is that we're both pretty nice."
The actress, whose new film "The Kingdom" is due out this month, also credits Affleck's ability to remain calm as another key to their happy home life.
"(Ben) is not someone who's ever going to blow up on anyone," she said. "What I mean is, if he's ever angry with me, he doesn't act out on it in a weird way or yell at me. And I am the same. So we can handle conflict in a very loving and adult way."
People reported that Garner also recognized both cinema stars' love of family, particularly their 22-month-old daughter Violet.