Grug is unhappy with the tribe's chieftain, who takes all the good women and feeds his sons the best food. Grug genuinely desires to change the status quo, becuase Grug is programmed to genuinely believe he's doing good so as to be more persuasive to the rest of the tribe. Grug overthrows the old chief and takes control. Of course some priveliges of leadership are important if you're chief, you are the noblest so deserve the most wives. Your sons deserve the best food... It's survival strategy.