Why are some friends brutally mean to each other?

My teenage cousin was roasting my dad so bad but my dad came with a comeback. They were going back and forth at each other. I don't understand how my dad isnt hurt about this. I would have been hurt. She insulted her best friend and her bestie got her back. Man teenagers are unapologetically brutal to each other. My dad told me those are just words and dont take it too offensively because I do.