His story of a Bulbmin's life, which doubles as nightmarish.
According to his 50 Facts video, he has four siblings that died as miscarriages, and he himself almost died from an allergic reaction at a young age. He was also bullied in school, and the bully died before finishing high school.
And according to this video, Chugga almost got on the Thunder Mountain roller coaster that derailed in Disney Land, killing a person and injuring 10 other people. He's had a fear of roller coasters ever since.
Especially keep in mind how he was planning on riding in the front, which is where most of the people died. He was one ride away from likely dying.
And remember those hilarious moments in his Sunshine LP? Like the trouble he had with the watermelons, or the really annoying level that he completed right before losing his save data from a power failure, or the huge annoyance the final level's blue coins were? Not so funny when you consider he was the one playing during that, in addition to the problems he was already having. Somewhat surprising he didn't have a Heroic BSOD by the time it was over.
Chugga has links to the alternate scenes in Majora's Mask that play when you fail something. Throughout the alternate scenes, he's completely silent, and the description of the video is almost never more than the affected person/thing's name, followed by an ellipsis.
In Super Paper Mario, during the scene where Luvbi turns into a Pure Heart, which is already enough of a Tear Jerker, Chugga stays completely silent, not even reading the text out loud. Then, once the scene ends and he gets the Pure Heart, he doesn't even yell "YOU GOT A PURE HEART! END OF CHAPTER!" like he always does. He just stays silent, quietly reads the text at the very end, and then ends the video there without even any closing words.
He does the same during Xenoblade Chronicles when Fiora dies.
In episode 52 of his Okamiden Let's Play, he stays very quiet as we watch the last moments of the Gorreo. Then, the picture at the end of the video is the interior of the Gorreo, which then morphs into the Ghost Ship.
The ending of episode 65 is arguably even worse, as an adorable picture of Chibi licking Kuni's face morphs into Akuro superimposed over the latter's face.
The other Wham Episode, Chapter 18. It ends with the sad part of the flight music from that chapter. A translucent image of Palutena is slowly fading in, and she repeats her (or rather, the Chaos Kin's) quote, “Everyone changes, Pit. Even gods.”
Any time Chugga finishes a playthrough could qualify, particularly if it was one of your favorites or if the ending was sad [Mother 3, Super Paper Mario, Okamiden]. The way he talks about how much fun he had with the playthrough...
In the finale of Link's Awakening, he talks about how meaningful the game, especially the ending, is to him, as someone who suffers from nightmares rather often. After that, he viewed the ending for the first time and it made him love the game even more.
At one point while making Xenoblade, Chugga became ill, and his sickness got worse and worse for over a month and a half. After 45 days of no updates, he made a video explaining it. He doesn't know when he'll feel better, and until then, he can't make videos at all. It's utterly depressing, not just for his viewers who were looking forward to watching Xenoblade, but for Chugga himself, who feels as though he has no energy...and considering how energetic he usually is, that's honestly heartbreaking.
Especially if you listen to how tired he sounds, when compared to how much energy his voice usually has.
Staying with the tradition of staying silent after major events, Chuggaa is completely silent after Fiora's death in Xenoblade, even after a new mechanic that he would normally explain is introduced involving the Face Mechon. The ending shot is also Fiora fading out to represent their death.