Happy endings are a rare delight in comics, serving as a sort of treat for wading through the sea of dark or ambiguous endings generally found in the industry. Arguably, they're so unusual that even endings with dark undertones can appear happy, solely because they aren't as depressing as so many others.
And so even legitimately happy endings often aren't as positive as it would initially appear, because they tend to come with negative implications that go entirely overlooked in lieu of a sense of superhero-based closure.
While they vary heavily in how subtly they're implied, more often than not writers will sneak one or two concerning suggestions into their endings, possibly just to prove that in the comic industry, you can't have nice things. Better yet, sometimes a series will end in a way that's good for most people, but absolutely terrible for one character - and it's totally blown over, with the fate of this poor unfortunate seemingly entirely forgotten about.
And so, when they're not accidentally letting murderers go free, accidentally ending the world, or accidentally making everyone in Las Vegas be homeless, these endings are pretty happy - so long as you can ignore all that other stuff.
10. Superman: Red Son
Superman: Red Son's ending is one that is sure to have delighted many fans.
Not only did we get to see how fast technological advancement would be in a world where Lex Luthor became a legitimate leader, but we also got the delightful plot twist reveal that, in this universe, Superman comes from the future, and is sent to the past just before the Earth explodes.
It's such an exciting ending that it totally distracts from how sad it is, as the implication is that Earth is permanently destroyed, and that Superman cannot fix it as he is merely part of a time loop he is unaware exists.
This means that the ending of Red Son is less exciting and happy for seeing how Superman came to Earth, and rather a tragic superhero sequel to Groundhog Day - which admittedly still sounds pretty cool.