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 cliché. But that doesn't mean it stopped being fun!
Spider-Girl #16

The necessities of the format mean that the game must constantly be contriving reasons for two characters to fight. But about sixty to seventy percent of the fights are founded on misunderstandings or pranks so that after the fight ends, the participants can be all chums again two dialogue lines later, as the winner awkwardly helps stitch up the loser's gaping stab wounds and picks up all the fragments of jawbone that blasted out of his asshole.

"I know it's practically a requirement in situations like these... but is there any chance we could pass on the gratuitous fight?"
Nate Grey/X-Man, The Amazing Spider-Man #420

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...
Sonic the Hedgehog/Mega Man: Worlds Collide, "Short Circuits" strip.

"Sometimes when us super-powered types come across each other spirits can run high; misunderstandings can happen."
Wolverine, 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 gathered in the same place, they must fight with each other 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)

Wizard Shazam: Henceforth it shall be your sacred duty to defend the poor and helpless, right wrongs and crush evil everywhere.
Linkara!Shazam: Also to constantly get into fights with Superman when the writers want someone who's a match for him.