Kristen Bell and Dax Shepard have to ''work really, really hard'' as a couple.

The pair have been together for nearly 12 years and have two daughters, Lincoln, five, and four-year-old Delta, but they admitted that they're ''pretty much opposites'' and have to be patient with each other as they're both ''painfully stubborn''.

In an interview with People magazine Dax, 44, said: ''We definitely had to work really hard at being a couple because we're both incredibly, painfully stubborn, and we're pretty much opposites.''

The couple got engaged in 2010 but vowed not to tie the knot until same-sex marriage was legalised in California, and though they wed in 2013, the 38-year-old actress admitted she had accepted it was unlikely they would ever get hitched because the 'Parenthood' star didn't see it as important.

She said: ''He has a great argument that the state having a piece of paper doesn't mean he's going to be nice to me and by my side for the rest of my life. [Rather], that is going to be evidenced by how we treat each other and the commitment that we make.

''I had surrendered, like 'Okay, well, I'm never going to get any sort of traditional marriage out of this, and that's okay because I trust him.' I really, really trusted him and believed that we were going to go the long haul.''

Dax then explained that he decided to propose as he wanted to make the 'Good Place' actress happy and wasn't worried about the ''tradition or history of marriage''.

He said: ''Ultimately, I was like, 'Well, I'm doing it because my partner wants that'. Forget the tradition or history of marriage as a concept, you knowing I was doing something that I didn't want to do because I loved [Kristen].''