What's so important thing in relationship?Watch
when dating - trust/passion/excitement/comparable personalities
when you fully commit (marriage etc) - the view of yourself as a single thing. You are together entirely, fail together, succeed together, you are not individuals any more but a single family unit.
The last one is the one we are loosing at the moment, and its the on that causes a lot of avoidable devorces. Two people who get married but still think individually - they think 'my partner is not doing X, so my life is not right' - they view problems as individual issues, and success and achievements as individual. Its a recipe for disaster in most cases. I don't blame the marriages that break - they exist inside our wider culture which is ever more individualistic by the year.. they are just products of the culture we live in. But you need to fight against it when you get married, and you really need to push the line that 'we are X family, we work together, pool everything, are responsible for everyone, support everyone, we fail together, succeed together, make mistakes together etc'