It is a theory proposed by evolutionary psychologist Geoffrey Miller in The Mating Mind. He cites that there seems to be a universal pattern to male/female short-term courtship; as well as long-term commitment. He proposes that humans employ the "scheherazade strategy" for long-term commitment, which typically extends for 7-10 years before running its course. The scheherazade strategy is the idea that females have evolved a more verbal proclivity to men because they need to keep a man engaged and interested in more long-term sexual scenarios. This is in line with a lot of psychological research that demonstrates that females start and carry most conversations in long-term relationships; whereas men shoulder this burden during courtship.

Furthermore, I also think this theory is logically consistent. It would make sense that there is an evolved strategy that lasts 7-10 years. Evolution is never wasteful and 7-10 years is probably the minimum amount of time necessary to raise a human child at rates successful enough to effect natural selection (and hence favour the selection of pairbonds that can remain stable for 7-10 years). After that, what does evolution care about your pairbond? Energy won't be put towards the stabilization of the bond when it is non-functional.

That is why I think the only actually happy long-term exclusive pairbonds are between people who would be really good friends regardless of sex.