In marriage, for one of us to succeed, the other has to succeed, too. That's something that couples who have mastered living together cooperatively fully understand. It's also a relationship lesson that some people never grasp.
Many years ago, when I was a sixth-grade public-school teacher, I used an exercise with my students that beautifully points up the difference between those who get it and those who don't .
In case you ever want to try the exercise with kids (or adults?), it goes this way: Assuming you're working with a group of 10 kids, you make 10 identical simple puzzles out of cardboard - maybe 12 pieces/puzzle. When you are finished, you take those 10 identical puzzles and mix them all up. You put an identical number of randomly-chosen puzzle pieces in 10 envelopes and give each kid an envelope.
None of the kids can assemble the puzzle with the pieces that s/he has received, since the puzzles were all mixed up. To complete the puzzle, each kid needs some different pieces. There are rules: You can't speak. You can't take, and you can't ask for what you need. In fact, you can't get anything for yourself. Instead, you have to give other people the pieces that they need. And when the others that you are giving to notice your need and respond, you get your puzzle completed.
When everybody works together, everybody gets the pieces s/he needs, and all the puzzles get assembled - because each person focuses not on "what I need" but on "what you need."
In my experience, most kids relatively easily learned to focus on others' needs rather than their own. Some kids couldn't do it, however: Prohibited from asking, demanding, or taking, they simply sat there. Completing the loop - meeting my needs by attending to yours - was beyond them.
Sometimes I think I met some of those kids later on in my couples counseling practice.