The not so common aspect about the relationships is the need to love someone, express affection, express concern etc. By calling this uncommon, I am not saying regular relationships do not have it. Its uncommon in the sense that its not the primary motivating force in most common relationships.
So, when people want a partner because they feel they will be complete only then, there will be a constant trying to fit the partner in a mold, such that the partner fills the incompleteness in them and its next to impossible to find a perfect fit. And this is a cause of unhappy relationships. Some relationships fit approximately, and then people are willing to just go forward with it and live with it. But, then there will still be constant drama - low level to high level.
I think the burden of incompleteness lies with the individual. I feel I should not dump my feeling of incompleteness on someone else and that I should fix myself first. I should put myself through rehab and cure myself of this perceived incompleteness. Then, when I get into a relationship, Ill be able to love the person totally without any requirement of me to get the other person to fill an emotional incompleteness in me. I feel, such a couple, where the partners are both not trying to get the other person to fill an emotional incompleteness in them are the primordial couple - Shiva and Parvathi.
A beautiful cartoon about the same topic - Missing piece meets the big O
PS: Post copied from a google chat and hence sentences might appear a bit odd :)