Quotes: Let's You and Him Fight

Spider-Girl: Isn't the whole Hero vs. Hero thing a tad played out?
Stinger: Oh, it's a total cliche. But that doesn't mean it stopped being fun!
Spider-Girl #16

The story's just a way of stringing a few fights together, I suppose, but the fights are mortal enemies dueling to the death only half the time. At one point, Jax has a fight with erstwhile ally Johnny Cage just because he's being a bit of a twat. Two questions: If this is just a friendly squabble, why can the plot only continue if I win the fight? And why did I just break his pelvis in half?
Zero Punctuation on Mortal Kombat

Orbot: Ah-ah-ah! You have to follow the crossover by-laws!
Sonic: We have to fight each other first? Really?
Mega Man: Aw, man...

Sometimes when us super-powered types come across each other spirits can run high; misunderstandings can happen.
Wolverine in the prose anthology X-Men Legends

Thorb (Bubbly): If we are heroes, why must we fight amongst ourselves?!
Iron Rabbit (Jimmy Five): Editowial twadition!

Marvel Rule #178: "When two or more heroes are together, they must fight for no apparent reason."

Superheroes only fight supervillains! (gets smacked in the face) ...Of course, if you wanted to become a supervillain, I think we could have that arranged.
The Tick, getting in a bar brawl with another bug-themed hero, "The Tick vs. The Tick"

Jafar: Tell me, Hades; what happens when one hero fights another hero?
Hades: Mayhem, uh, chaos, uh, collateral damage? Nice, but, ah, how would that start?
Jafar: Welll, what if Aladdin and Hercules were to have some terrible misunderstanding? With the proper motivation—
Hades: Oooh! They would fight until they destroyed each other. I like it!
Hercules cartoon

The game they played was called Let's You and Him Fight.
Shadow, American Gods (quoting J. Wellington Wimpy)