I honestly believe you can never tell if a relationship is going to last. In my own marriage which is going on 14 years I don't think of it as 'I'm going to be with this person forever.' Instead I think of more like 'I'll probably be with this person for the next six weeks. Then I'll re-evaluate.'
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I don't know why but the older I get the more interested I get in my parents' marriage. And it's interesting to be married yourself too because there is an inevitable comparison.