To Play or Not to Play That is the Question?

My sons have very active imaginations.
Robin tends to be my little warrior out to kill monsters and evil doers. A true hero in the best sense of the word. Robin loves to be a hero but for him its a bloody battle filled with taunts and battles. Robins propensity for violence is so opposite of me and to how we are raising them it baffles me.

Lance is my fantasy boy he loves to pretend that he is  the best of many things. Some days he is the best chef, other days he is Nigel Marvin, the greatest naturalist ever in his eyes. He tries to be the charecter he picks and he always wants to be the best.  He sufferes from a infiriority complex of galatic proportions.  To be Lance is not good enough.

Lance and Robin have a curious relationship, they  are so unlike myself and my siblings that they leave me frustrated and confused.  Growing up my sister and brother and I were well behaved, we didn't fight, No Hitting each other.  We went everywhere together, heck Pete went on my first date with me..LOL.  Flora and I were confidants from the start although I didn't give her much choice. When she came home from the hospital the first night I sat next to her bassinet and whispered my secrets to her. We still tell each other everything we depend on the other to be there, and we are there all the time.   We are very close, even now we all live together and we talk every day. We fight but its tempered with love and mutual respect.  I am the oldest and Flora is the middle and Peter the youngest ( I suppose at 26 calling him the baby is pushing it LOL)  There are 8 years between each of us. We played together, we watched out for one another, and we backed each other up. We may have yelled at each other but we never hurt each other.

I can't say that the same is true for Lance and Robin, Lance has hurt Robin.  Robin has hurt Lance.
Its lopsided more of Lance hurting Robin than the other way around. Robin will take more abuse before he has had enough and then retaliate. The idea  that he has to defend himself against his brother really bugs me.
I tell them no hitting, no hurting your brother. No No NO .

True if someone tries to Hurt either of them the other will come running to his defense. I have had proof of that as Robin has punched a boy much larger than he is, and older and gave him a bloody lip,  for picking on Lance. Lance has shoved a girl out of the yard and told her never to come back because she made fun of Robin.  Now Robins retaliation was more violent, but Lance was up against a girl. Yes before you ask we subscribe to the old fashion notion of you don't hit girls.

If you watched my sons argue and fight through a day you would think they watch people fight all day. That my husband and I argue constantly. In truth in the 20 + years we are together we have had loud arguments only about 10 times and I am adding a few as a cushion because I think its even less than that.  We disagree but we don't fight about it.  We go off think about the others POV came back and discuss things.  Its one thing to say Italians are loud and argumentative and Puerto Ricans are the same as a collective people but our mish mash of a family just doesn't argue like that.

We are loud yes.
We like loud music,  loud talk and loud TV.
But we don't fight.  SO WHY DO MY KIDS FIGHT?

Why can't I teach them to get along? Why is what came so naturally to me and my siblings, such a big leap for my boys?
They can't pick a game to play co-operatively,  they cant talk about the same things without fighting. They argue about everything from TV to sleep.  What the hell am I doing wrong?