simple statics question

why is it when you stack two blocks on each other that the top box with mass [itex]M_t[/itex] has a normal force of [itex]M_t[/itex] and a gravity force [itex]M_t*g[/itex]where as the bottom box has a normal force of [itex]M_t+M_b[/itex] where [itex]M_b[/itex] is the mass of the bottom box yet gravity is [itex]M_b*g[/itex]?

i guess i don't get why gravity is the same for each box the the normal force considers all the weight stacked on top (why doesn't gravity?)

thanks for your help!!