I'd say it's not that people don't want to date people that are far apart in attractiveness, but rather that everyone wants to date "up" or "across". And since

I'd say it's not that people don't want to date people that are far apart in attractiveness, but rather that everyone wants to date "up" or "across". And since if you go "up" the other goes "down", it tends to be "across". Of course this assumes that attractiveness is objective, and purely physical. Which is false in both counts, but it's a simplification as we all know.