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Tear Jerker: The Tyrant Falls in Love
Volume 6: After Tomoe and Kurokawa announce their relationship for the first time to Souichi's dad, who accepts them fairly readily, Morinaga slips off in the commotion to be alone. Souichi finds him crying in the bathroom, because Soujin's understanding only threw into relief how Morinaga's own parents rejected him after they found out he was gay, parents whom he's not even on speaking terms with anymore. Though he otherwise tends not to make a big deal of it, it's obvious in this moment just how much he cares, and it's hard to resist crying a little with him in sympathy.
Chapter 9 of Volume 7. Period.
When Souichi and Morinaga make love after Souichi's house burns down in Volume 4. Probably the cutest thing ever is when Souichi gently nuzzles Morinaga's arm injury, which he'd received earlier trying to defend Souichi's little sister.