i mean he kind of spells it out but it's primarily because shion is someone who defies his expectations. like shinonome sees the world as a plate of pristine ceramic but any little crack or divet that he couldn't predict is interesting to him, and shion just goes the extra mile by picking up the plate and shattering it on the floor. it was definitely more of a cat and mouse interest initially but then shion fought back and carved out his own future and shouma was thrilled. shion's the one person who can fight him on his terms and win, and on top of that shion ended up saving him from death which only surprised and delighted him more. they couldve written their relationship on slabs of gold and sold them for millions and instead we got it in stride gate for Free