Trent to dad- why is brother petersen's last name petersen? Brett- well, because his dad's name was Petersen & his grandpas last name was petersen. Just like your grandpa's last name is messer & my last name is messer. Trent- well, someday my last name will be Johnson. Brett- Johnson? Trent- ya when I marry Lily. Brett- No then her last name will be messer too.