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Tear Jerker: Game Grumps
"Ode to Jon". It has its funny moments, but it's still rather bittersweet, seeing Jon suddenly leave.
It's a depressing sight watching everyone huddled around the empty seat on the couch that Jon used to sit on, a pillow and a picture of Jon on the seat, with Arin crying and rubbing the empty seat as though a close friend had died.
In-Universe: Sad Hoshi.
At least until it turns out he's a pedophile.
All of The Wind Waker: Episode 19. Essentially the entire episode revolves around Danny revealing his past where he struggled with Mono, depression, and OCD.
Danny seems genuinely sad when he realizes they've finished Kirby's Epic Yarn, especially during the scene when Prince Fluff says goodbye to Kirby.
Jon reminices about his time with Danny, and how the two used to laugh about Wolfjob together.
Jon: I miss Dan. I really do.
Many fans feel this way about the longest playthrough the Grumps ever did, Sonic the Hedgehog (2006). It was done in by Jon's departure and Left Hanging at part 108. It stops at the very end of Silver's campaign with the Grumps about to beat Iblis's final form, and they had already cleared Sonic's and Shadow's campaigns, which would have launched them into the Last Episode to beat Solaris in an epic conclusion to the game. Jon himself had said it would be an incredible finale... then left Game Grumps. Afterwords, Arin said that the playthrough was over for good, because they had gotten to a point where there wasn't much more to do near the end, and Danny felt he wasn't there for the journey and it would feel like an intrusion to pick up where Jon left off and continue only a smidgen of the game.
In Part 13 of Katamari Damacy, Dan recalls his first day of nursery school. Mixed with Heartwarming, he says how he cried after saying goodbye to his mom, and was so fussy as a result he threw blocks at kids.
Shadow of the Colossus naturally brings a few of these out, as Danny keeps saying how bad he feels about killing the Colossi.
Agro's death. Danny is "so sadmad right now."
Not helped when he reflexively whistles for Agro after defeating a Colossus... and she's not there. Danny accidentally makes himself sad.
What makes it worse is that Arin managed to trick Danny into gushing about Agro right before her death.
During the whole ending, Danny seems like he's about to start crying, and even begs Arin to cut out his shtick of pretending he doesn't know what's about to happen. He ends up declaring that he might need to take a couple weeks off before he's up to doing the next game.
Just the entire implication that Jon and Arin had a fall out adds more sadness to the fact that they seemed like such great friends, many fans even considered them brothers. To see so much dishonesty and any lack of information about what happened (which has frustrated fans alike about Jon's abrupt departure from Game Grumps) is basically this.