I think because both Hotsuma and Shuusei feelings are reciprocated. While Yuki and Luka you can say that its not to that extend yet, because Yuki didn't remember their past and assume Luka is sort of his dear friend that he wants to protect. I still don't understand why Luka didn't want to tell anything to Yuki about their past. Maybe it has the connection with what he says to Yuki that the one 'he's looking at' is him not 'her' but im desperate to see the moment Yuki knows his past. It will be burdensome for sure, because all dark stuff of 'her' will be revealed
yep. and then again sairi gave a hint than past-life yuki can't go out or meet anyone freely (except maybe luka). but I don't think the way they act will be so different with now if yuki remember. they just not a type to conflict or bicker like hotsuma x shuusei. so I found it more amusing when shuusei act sweet and then hotsuma goes tsundere (▰˘◡˘▰)
I always thought that although luka x yuki constantly shows affection each other everytime (especially luka), I think the most sweet and honest couple are hotsuma x shuusei pair. they act like married couple, they bickering, yelling, but they show the importance of each other is still above yuki rather clearly than others. my favorite no doubt is luka, but my no.1 OTP will be always hotsuma x shuusei ε=ε=(ノ≧∇≦)ノ