I can see it clear as day- he's been acting more and more like Kazuki, without realizing it, and it's really hard on Suzuka. She might even feel like history could repeat itself if he makes a declaration like that- that, somehow, him saying she can give him an answer to his confession by the time he becomes the fastest in Japan, just like Kazuki, means he'll be cursed to die soon, too.
Of course, it's probably just the fact he's reminding her more and more of Kazuki, and that alone is painful. But still, the possibility of her feeling like a curse also exists.