Maybe that's right. Lol I thought Mitsuki was the lover of the grandpa too but nope! I'm glad I was wrong. ╮(￣▽￣)╭
mitsuka vanishes once that the wish of the person has been granted, it was like he was going to be reincarnated again but in their case the two of them love each other so when mitsuki in the present time reincarnated again he still remembers his boo... and good thing because they also have somewhat connection in their dreams so they were able to communicate
Ohh it's a bit more clear now.
Tell me if I get it right people. So, Mistuki is sort of like a genie or smth, because in parallel events, his grandpa used their meeting in a dream to be honest of wanting to have business and pursue the wife that he wants for himself— and he did. On the other hand, Mitsuki granted Sousuke's words saying 'I want you to fall in love with me' and after some circumstances, they ended up together.
Because the whole time I thought that Mitsuki is an ex-lover of the grandpa, and reincarnated in Sousuke's time, but Mitsuki is a material for dream catching.