Just like being jerks, heroes quitting is an immediately compelling situation because it's fundamentally against character. So once again, savvy artists used this gimmick a lot in the 50s and 60s to make readers do a double take at their issue - "Superman has quite being Super? What the deuce!" they'd say, before throwing money at the cover. Generally it was the most hot-headed member of the group like Wolverine or the Human Torch, and of course, just like with Superman being a jerk, this kind of diminishes the heroism of superheroes since it implies it doesn't take much for them to quit. One would assume they'd be more legit. Too legit, even. Example:HawkeyeThe Avenger with the most instances of hitting and quitting is Clint Barton aka Hawkeye - he literally has other superhero identities like Goliath and Ronin just so he can quit as much as possible. Now, I like Hawkeye - he's a pretty fun character, but with that being said; nobody on the Avengers should care about Hawkeye leaving. The dude shoots arrows, and offers little else. In addition, why did he break his most-likely-very-expensive bow? That isn't a requisite to leaving a team: do cops disassemble their pistols when they leave the force? Do basketball players deflate all their balls once they retire? Dumb decisions like that is why they don't need him anyway.