Same thing that makes a good marriage at 18. Both parties need to understand that it's basically a partnership, a business, the product you're producing is children. It's not about hot sex, or love or companionship, it's about pooling your resources to give your kids the best chance for success. The only thing that changes is that after 45 the children grow up and leave, but if you're had years of operating under the cooperative mindset you'll be fine.