Why is my is my17 yr old brother shy of my mum,?

my brother is worse than girls,his shy with every 1.even my mum and dad.i am the only 1 thats knows his normal self,with me his the bigest holigan in this world.and a pain in the bum.but my mum thinks hiz a angel,coz front of her,he is so inocent and shy and doest express him self.his known my our mum all our lives.and shes always been here for us