> "Two people get married because they love each other, but who can guarantee that they’ll only love one person their whole life?"

Nobody can guarantee that, and it actually seldom happens.

Therefore, it's quite childish believing in "one only true love", and we should better face reality: humans beings are not much suited to long-term monogamy.

Can one live happily monogamous for life? Some can, but the vast majority simply aren't able and won't do it.

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BTW, in no case "all end up the same way": life is too much varied and unpredictable for that to happen. Such broad generalizations are always untrue.