let me list every sad, emotional, and tragic event in the game in order of when they happen
The kidnapping of Bowser's entire army, Bowser, and Peach
The slavery stuff in Chapter 2
Tippi's surge of trust in you
Luigi being caught by Count Bleck's army and turned into a villain
Prince Squirps talking about the good deeds of his presumably dead mother
The Floro Sapiens going crazy because of the polluted water
The destruction of an entire dimension, Sammer's Kingdom
Luigi's death
Mario, Bowser, and Peach's deaths
(they're all brought back because they're the heroes of prophecy)
Luvbi's death, followed by her father reminding you that you cannot fail your journey, and you must not let anyone else ever feel this pain
O'Chunks and Bowser's deaths
Peach's death
Luigi's death
Count Bleck and Tippi remember who they truly are, but it's too late for them to love again, because...
Count Bleck's death
aaaand finally, THE END OF ALL worlds