The only reason, unquestioned and uninterpreted, why people of the opposite sex get together and live together, have feelings for each other and become emotionally close, is the instinct to reproduce. It takes three to five years to reproduce and raise a child to the age at which he already acquires some kind of independence. Accordingly, this is the length of time for which the relationship is originally designed. That is why we often encounter cases of "serial monogamy": when one partner after a certain period of time changes for another, and so on to infinity.
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