This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Calvin and Hobbes
Some of the Little Known Facts that Calvin's dad recites are quite cruel. He claims that while most babies are bought from Sears, Calvin was a Blue Light Special at K Mart. Also, he tells Calvin that he grew from a grub and pupated at age two.
One of the earliest story arcs of the strips deals with Hobbes having been stolen from Calvin by a dog. Calvin's desperation as he searches for his best friend really hits home, especially to those readers who had beloved toys lost and stolen as kids. It's also one of the first times in the strip Watterson received fan mail from readers genuinely concerned about Hobbes' wellbeing.
The fact that no one is ever happy during family vacations except Calvin's dad. Calvin's miserable by the end of it, Mom is a grump without her coffee and being woken up at dawn, and Calvin's dad keeps taking them to various islands for camping. (Bill Watterson later confirmed that his dad would do the same thing.)
The last vacation story is arguably the worst. Prior to this, Mom and Calvin at least begrudgingly joined in on camping activities to spend time as a family, but at this point, they're not even trying anymore, and pretty much go out of their way to tell Dad how much they hate the trip AND his hobbies. The guy works his ass off in a boring white collar job day in and day out to provide, and his family cant even give him one week of mild discomfort to indulge in his own passions. Granted, camping isn't great if you aren't interested in it, but it's hardly the ordeal that Mom and Calvin acts like it is.
Though considering the utter disaster their first trip (where it continuously rained up until the moment they left) was, it's not entirely unsurprising. Hell, Dad's utter disregard for his wife and son's own feelings on this can be pretty mean on his end.
Stupendous Man never wins in any of his storylines. Calvin tries to claim that he gets "Moral victories".
Also, compared to how Calvin normally behaves, in one arc he changes into Stupendous Man to try and pass a test at school. If he had simply told Miss Wormwood what it was, she may not have dragged him to the principal's office and gotten him into trouble since he technically didn't cheat.
Rosalyn and Calvin never get along, due to Rosalyn starting her babysitter stint by locking up Calvin in the garage and then in his room. Calvin then retaliates, and they never really find common ground... until he teaches her how to play Calvinball.
A storyline has Calvin finding an injured baby raccoon and trying to save its life. He doesn't succeed.
Calvin: He was just little! What's the point of putting him here and then taking him back so soon?!
Calvin's dad has to break the news to him. He's not happy about it, especially when Calvin starts to sob loudly and says he just can't believe the raccoon is gone.
Hobbes and Calvin, while talking about it, promise that they're not going to leave each other.
This story has a bit of Reality Subtext to it, as Watterson said it "just wrote itself" when his wife found a dead kitten one day.
For reference; most boys Calvin's age would poke a dead bird with a stick. Calvin waxes philosophical about it instead.
Another bit of Reality Subtext. The dead bird sketched in the first panel was one that Bill Watterson found outside his home.
The one where Calvin and Hobbes figure out that they can play together all night by dreaming about each other, then go to bed saying they'll see each other soon. Seems more like a Heartwarming Moment, until the 10th Anniversary compilation where Bill Watterson revealed that he wrote that strip after the death of his cat Sprite, who was the major inspiration for Hobbes' look and personality. "We can always meet again in dreams."
As sweet as a plum, as lovely as dawn, rolling its tongue over its gums:
My tiger and me, as happy as could be, sat out on the porch as the whole of the sky clouds quietly over.
And it starts to cry, softly, on my shoulder.
We don't want to grow up, but we have to grow up. As sad as I am, I do understand. I do understand, it just makes me sad.
My tiger, my heart — We're growing apart. We're trying to be friends, but it's hard sometimes to be friends with something that eats butterflies and pencil sharpeners, and I think it would be happier being free...
My tiger, my friend, my little godsend. I know someday we'll be happy again.
"A man's home is his castle, but it shouldn't have to be a fortress."
Later, he talks to his wife about how, during his own childhood, he always trusted his parents to fix everything, and it never crossed his mind that they might not have any idea how...which is the same situation he's in now.
Meanwhile, Calvin is practically in hysterics trying to find Hobbes because he thinks his best friend has been tigernapped. He even tells his mother that "Hobbes is so trusting," especially since Hobbes is established as a misanthrope. (Thankfully, he was not.)
"Mom says Hobbes wouldn't have been stolen because he's not valuable. (sniff) Well, I think he's valuable."
Calvin's mom's take on the break in.
"This is something that you always figure will happen to someone else. Unfortunately, we're all 'someone else' to someone else."
The baseball story arc. Calvin signs up for a baseball team due to being bullied by Moe for being the only boy who didn't sign up. However, close to the end, Calvin accidentally catches the ball for the wrong team and makes his team lose. All of the other players cruelly and mean-spiritedly insult Calvin for an understandable mistake, in a way that really hits close to home if you've been bullied yourself. (One of them even asks the coach if he can hit Calvin with the bat.) The kicker? Due to this name-calling, Calvin asks the coach if he can sign off. The coach's response? "Okay, quitter! Goodbye." Thankfully, Hobbes has a better idea when Calvin gets home.
There's also the story arc when Calvin has to tell his father that he broke his binoculars, and Dad flies into a tirade lasting for almost an entire strip. At the end of it, a tearful Calvin looks up at the man and says: "I have an idea. Let's pretend I already feel horrible about all this and that you don't need to rub it in anymore." Even Dad looks heartbroken by that one.◊
What's worse is that throughout the story up to that point, Calvin is genuinely worried about how angry his father will be, and it almost makes the reader wonder if his fear is just making things seem worse in his head. It's not.
Many of the heartwarming scenes between Hobbes and Calvin become outright devastating if you agree with the interpretation that Hobbes is only alive in Calvin's imagination, and that that's all he has.
Ka-ZAM!◊ Watterson comments in the tenth anniversary book that "imagination is not always appreciated."
Also, Hobbes shows No Sympathy as Calvin, still sick, lays awake in bed fretting about if he'll go to the hospital or if he's dying. To be fair, Hobbes himself doesn't want to catch Calvin's bug too, but still...
Susie wants to be friends with Calvin, or at least play with the only kid on their neighborhood block. Calvin refuses, due to Susie being a girl. He has made her cry, tossed apples and pine cones at her, and refused to play house her way. While he is sincere at times, like when he thanks her multiple times for finding Hobbes and keeping him safe, Calvin is too mean the rest of the time.
Poor Calvin's endless misery in school, especially considering that he's in first grade. Middle and high school is going to destroy him. What makes it worse is that Calvin does enjoy learning, but can't handle the stifled public education system, which is based on boring repetition and test taking to churn out mindless worker drones.
Many people who grew up with undiagnosed learning disorders like ADHD have commented that Calvin's school experiences remind them of their own. With that in mind, Calvin's behaviour at school becomes heartbreaking: he's not acting out for the sake of it, he's trying to learn but just cannot function in a normal classroom. If someone actually took a moment to help him, he might do okay or even brilliantly, but everyone just thinks he's badly-behaved.
The legal battles that Watterson had with his syndicate about merchandising Calvin and Hobbes, as he detailed in the tenth anniversary book. Although he could have used the money, he said that making Hobbes into a plushie or putting Calvin's face on a coffee mug would destroy magic of the strip. Especially sad since this resulted in those knock-off window stickers of Calvin urinating on things, which should have been a blatant trademark violation, being perfectly legal thanks to vagaries of US copyright law.
Watterson having to fight for his comic to end on his terms.