Why do men feel the need to show off in from of one another?

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This morning i overheard (i wasn't eaves dropping, they were talking loudly and i was in the same office trying to work!) my boss boasting to some male colleagues about how he'd beat up some blokes at a pub once. This kind of boasting and showing off often happens in my workplace as it's male-dominated. So it got me thinking, why do men always have to boast and show off in front of one another? It didn't impress me one bit, in fact it was annoying me as i was trying to work!