Sharing life together, doing things and events together that you both like, talking and communicating on a daily basis, keeping the sex life fresh, always being able to trust one another, make plans for your future, accepting each others differences and opinions.
Quantity time and quality time.
I feel strongly about this since this is my problem. They MUST do things together as hobbies. Golf, wine tasting, or shopping. It does not matter. My SO and I have two different due to totally different interests that we have acquired. The only thing we have in common is coffee!
I squeeze a raquetteball....