Many Relationships fail is because almost everybody wants a mate they can look up to. You want someone who you feel is better then you so they can make you a better person.

Look Up To The Other and the Relationship Should Work
The problem is that if you look up to your partner, then your partner probably looks down on you. There can' be an equal and respectful relationship with someone you look down on. So the secret to a good relationship is for both partners to look up to the other. If they both think the other one is 'better' than them the relationship should work.

Which are the best relationship on a long run?
The best relationships are the ones where both partners look up to each other. This remindes of a great quote:
"Love isn't about looking into each other's eyes. It's about looking out, in the same direction." Maybe it goes beyond ...everybody wants a mate they can look up to.
to the point that we want a partner and relationship that makes us feel "complete." That can mean many different things, but it always means the same thing. We "need" the other person and the relationship works well as long as the person meets those needs adequately.

Relationship that makes us feel "complete"
The real tragedy is not the failed relationships when needs aren't met. The real tragedy is our own lack of self-sufficiency and achievement of potential. We should be learning to understand our needs and take sole responsibility for meeting them. Thus, the "ideal" relationship is one where neither person NEEDS the other in the truest sense. They WANT the other because that person is looking in the same direction. Maybe not all the time, but at least most of it.