#and the loneliness he experienced being left behind in the hosokawa clan after all the other swords were taken away or sold off

[ikusayu-no-hana]

the thing is. the thing about kasen's former master is. he was so possessive and jealous that he'd cut off people's heads for simply staring at his wife and wiped the blood on her kimono. he mercilessly mutilated her christian maid's face in order to discourage gracia's interest in the religion (since hideyoshi had outlawed christianity). he didn't want to keep gracia in hideyoshi's care for fear of hideyoshi developing an interest in her. when gracia's father (akechi mitsuhide) was declared traitor and the entire akechi clan was declared traitorous, tadaoki refused to divorce her, sending her away to the mountains. the eisei bunko (the hosokawa family museum) displays a set of karuta cards tadaoki made personally for gracia, with all the poems written by his own hand. her killing herself in order to not be captured by mitsunari's forces was the decisive factor that led to tadaoki siding with ieyasu against mitsunari. and after she died tadaoki commissioned a bell in the western shape to be made in her honour (in accordance with her religion). its all so messed up. so yeah its no wonder kasen is the way he is towards those he cares about--in hiden, he worries about shokudaikiri but his comforting words come out to be harsh (shoku points it out too). in giden he tries to coddle sayo and keep him away from the date swords so much so that sayo sort of...rebels. he tries to talk to ookurikara because fundamentally their loneliness comes from the same place but of course it backfires, their coping mechanisms are polar opposites. heck even his in-game sparring interaction with izuminokami is him being max tsundere.

tldr: kasen doesnt get what love is because he has never witnessed a healthy version of it.