Responses
Saying I love you isn’t always common in Japanese relationships and Sada isn’t the type to say it even if he could have. He shows it through his actions.
There is a comic about Sada on the authors twitter that isn’t translated here. Basically, Sada has never really loved anyone and as per the convo in the hotel has complex feelings about romance. The comic depicts previous relationships and he basically experiences domestic violence because he wasn’t outwardly affectionate and the other person felt lead on (and is obviously an unstable person).I think he attacks Sada with a knife.

Sada never said I love you back, and Mamiya never got to read the letter he left for him.