It's bad to mistreat your family.
Dave beats up Kyle and takes his things because he's the big brother, so he's in charge.
Dave wants to continue to be in charge.
Dave learns that you shouldn't mistreat your family, so he starts treating Kyle better.
Dave keeps taking Kyle's stuff because he's the boss and such.
Kyle forgives Dave, and they start having fun together rather than fighting.
Kyle decides to go to the gym until he's strong enough to fight back against Dave.