“Chemistry” might be a placeholder for something far more complex.
Have you ever thought about what it was that drew you to your partner to begin with?
We all handle life in society differently, so what we need to take into consideration is also that what attracts us to someone can be partially the persona created. There is, however, the essence of us that comes through stronger, especially as time goes by and in situation where we are forced to be the real us.
What exactly is that essence?
Who we are drawn to as friends is similar to what draws us towards a partner. Even though “opposites attract” is a real thing, it is far less common than deciding on someone with whom we can communicate well.
Google “people are drawn to partners with similar ancestries” to see just for how long it has been suggested that common ancestries can play a huge role in whom we get the right signals from.
Blood type compatibility is also huge and goes far beyond that. Just like mice seem to recognize the Rh factor in each other when mating, we also appear to just navigate towards those who have a similar blood type as we do.
All of this also plays a huge role when it comes to producing healthy offspring.
Nature knows. Now you do as well.
See also:
https://www.datebytype.com/blogs/view/137/What-is-blood-type-compatibility