It's absolutely cultural. Within Hong Kong culture (a close friend is Cantonese), there's this funny thing where people fight to pay the bill. You typically get points for doing it before anyone else even has the chance. Sometimes a person will get up to "use the restroom" then go pay the bill away from everyone else. It's seen as a privilege and an honor to be able to pay for everyone. Considering that in the family, we sometimes go out to seafood places with a dozen or more people (all family), it can get expensive. It's a bit of a show-off thing IMO, but the family is not obnoxious about it (no one actually tries the bathroom thing with us; they just joke about it...but we also never ever split the bill or even show it around or let anyone else see it).