I disagree with the overall premise. Been together with my partner for 22 years. There is definitely such a thing as being “right” for each other, having the chemistry, the fit etc. it’s actually hard to explain because you get into really vague concepts like soul mates etc. this is not to say that the relationship is alway cloudless but the overriding factor is exactly the opposite to what you describe - sometimes you do find the right person; the fit is there, period