I don’t understand why uke left the seme?

i think it was a mixture of having to leave because his father was in the hospital and also wanting to let go(?) of seme

Agree! There was a practical need, but i think the leave a note change your contact info extreme was because sawa wanted to move on. nana never stopped dating other people when he was sleeping with sawa and never said he liked him. When he had that conversation with the women from the ballet it clicked that their relationship would never move forward if only one of them had feelings. Since nana never indicated he liked him in more than a f buddy capacity- he assumed he was the one oar. Nana admitted he could have tried harder to contact sawa didnt. He also went and got married. That probably validate sawas feelings that when he was no longer easy seme wouldn’t bother.