What would you consider as a sad (comic/writing) idea?
Basically, it’s just a two-part thing.
One: You need a character who the audience sympathizes with and adores. You can create one — it’s your job to make them likable in that case — or take one from an existing fandom — then the likeability is already set for you.
Two: You need to take something dear or important to this character and do something bad to it. It doesn’t have to be something absolutely awful and irreversible, though the weaker is the audience’s connection to the character, the bigger is the impact needed to get a reaction. Like, if you set up the character well enough, you can make the character’s favorite teddy bear losing its button-eye seem like a tragedy!
That’s if you were asking for the ‘science’ behind how sad ideas work.
If you were asking what kind of sads and feels actually get to me then… uh… I have no idea. I have a pretty high emotional empathy threshold and I’m fairly desensitized to fictional awfulness, so… *shrug*