Some people do this without thinking. There’s no direct ill intent towards the other person talking. They hear someone else talking. It doesn’t interest them. They’d rather catch up with the person next to them so they start their own conversation.
There has never been a default rule at gatherings that only one person may speak at a time.
True. Sometimes I also direct the blocking conversation to a person sitting right next to the one that talks not-worth-hearing-crap to start a new conversation. If it weren't for that one certain person, I'd be happy to just listen and not have to make any small talk.
Me: "Wow this wine has notes of grape and oak barrel, are you getting those tasting notes too?"
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