it's something both their parents transmitted culturally/behaviorally because they themselves converged by living together

I very much agree and I think there is even more here: The people in charge of their education (mother, father) weren't "drawn independently", they are two people that chose and love eachother (at least for some time).

People who marry each other tend to come from similar cultural background, or they can share some interests. All of this can then be transmitted to their respective kids.

After all Jack & Oskar's parents probably both liked white jackets before they even met.