yup u're right .Jealousy always work. Yaoi world turns around with jealousy rule .like a leverage for tsundere ukes.
that would be such a good ending to the story because senpai finally admits that he loves him
He may not have said the word "love", but during their latest conversations, sempai told Morinaga he didn't ask him to stay beside him halfheartedly. Morniaga is the one with the problem, because he's using his own feelings/moods as a basis to measure sempai's feelings/moods. Since sempai is a completely different person, of course he's going to think and act differently than any other creature on earth.
I can only image that walking in on Morinaga having sex with another is the only way to shake sempai up.
Another guy should show, that is seriously in love with Morinaga. So senpai needs to fight for his man, and finaly be honest about how he feels for Morinaga.