It is rude to ignore other's accomplishments.
Bob and Steve talk bout things they've done, Bob talks about sports and Dave talks about chess.
Bob wants to keep talking about his acomplishments.
Bob takes turns with Dave talking about their accomplishments.
Bob thinks that his sports trophies are more important than Dave's chess trophies thus he downplays Dave.
Bob and Dave both have fun talking and listening as well.
Bob hurts Dave's feelings and others see Bob as a braggart.