
Let's put it this way Nanae and Ken already get in to what they want after high school and Taku is preparing for his path (College/University), Nanae and Ken are close friends that talks about each other's relationships, that's all they're not dating, just friends. Taku is thinking way too deep about their relationship because he thinks Ken is letting go off him, but that's not the case. ٩(๑❛ᴗ❛๑)۶
****Thats my understanding anyway****

No all that is Taku's assumption. He was the one that invited Ken and Nanae on the roof to watch the fire works, and to the lighting of the candles, he had is candle lit then Ken light his off of Taku's and then tells Ken to light Nanaes candle, because of what Taku thinks and what lighting of the candles meant he thought it was a great idea for both of them. Ken should of clarify things but he didn't, so Taku think that's what going on.
This page(linkbelow) should clear things up, the writing on the line is written by Ken to Taku in the Diary that they Both shared
http://www.mangago.zone/read-manga/sayonara_note/mh/c002/25/
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I tried!
I'm confused. nanae knows ken likes taku and she wants him to confess but now they are going out with each other and staying together? she didn't seem to like him before?