How does a relationship succeed?

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What factors in the end decide whether a relationship will succeed or end in failure? How can a heartfelt love be sustained over a period of years - such as in the tale of Jorinda and Joringel, of whom it said; "their greatest joy was in being together"? Is it more important to have interests in common - according to the maxim: "birds of a feather flock together"? Or should there be a certain differences, because, "opposites attracts"? Does the ideal lie in each partner having some personal space or "locked to each other" constantly? Is a romantic attitude realistically enough to gurantee, contrary to the generally accepted opinion, the initial feeling of love in the long run? What do you think, Give your opinion.