[mironicus]

But maybe, just maaaybe, Kodaka's narration on vol 8 epilogue that the story so far was only a prologue and it will change from unlucky slice-of-life comedy (I forgot his exact words >.<) to love comedy isn't all joking?

Spoiler:

If you interpret his last sentences right he already announced that he wants to marry Sena.

the story so far was only a prologue

What is the story about? It's about Kodaka himself and his life. He is the narrator after all. So replace "the story" with "my life".

My life so far was only a prologue.

it will change from unlucky slice-of-life comedy to love comedy

Means his sad life up until now comes to an end and a new part of his life full of love starts now.

And the first step to start his new life is to reply to Sena's confession. She is the only girl who asked him and so she is the only one he can reply to.

And of course, the love comedy can't even start if he reject her and the whole announcement has only this one meaning - he wants to marry Sena.

And he asked Rika for help at the end of Vol. 8 because he needs her support to make things clear to Yozora.