The psychology of romantic attraction is a topic that has arisen here before, but I thought I would share an idea of my own.
Healthy long-term relationships probably always need to be built on a basis of mutual respect. Many of us respect people who show knowledge, talent, or skill in a field that we consider admirable – from academic accomplishment to music to athletics.
At the same time, being in a relationship with someone in too closely related a field seems likely to cause problems for many people, as a consequence of inevitable competition. Two academics might find themselves feeling competitive about publications or grant money, for instance. While, for some people, that might be a spur to greater accomplishment, I think it would be more likely to be a source of strain for most people.
That makes me wonder whether perhaps we spend too much time looking for partners within groups of people overly similar to us. Students in the same program have social events together and meet through classes; people in the same profession socialize together; participants in the same sport meet during training and competitions. Less commonly, we spend time socially with those ideal candidates: people who are admirably skilled in an area we respect, but do not excel in ourselves.
I think perhaps aristocrats everywhere have learned this lesson. Aristocratic events (insofar as I know anything about them) do tend to mix together successful and influential people from many walks of life, from prima ballerinas to up-and-coming diplomats. It seems plausible that many successful romantic unions could arise from this.