"A lot of it has to do with age," Douglas, 69, who costars with pal Diane Keaton in the new romantic comedy And So It Goes (out July 18), tells PEOPLE in this week's issue. "When you've accomplished a certain amount in your career, you're not so focused on your ambitions. It makes you appreciate – and hopefully you do that sooner rather than later – the value of your partner."
"Like a lovely orchid, or anything else that's nurtured, marriage prospers and grows, but if it's ignored, it withers."
The actor also discusses being a father to his kids with Zeta-Jones – son Dylan, 13, and daughter Carys, 11 – and his views on being open with them.
"I'm more and more prone to tell kids just about everything," he says. "There's been a whole school of thought about things you should not tell children – if there's a problem in a relationship, [people say], 'Let's not let the kids know.' Kids know. They know everything. I would err on the side of pretty much sharing everything. They can digest it, you know? It's life."
For more from Michael Douglas, pick up this week's issue of PEOPLE Magazine, on newsstands Friday