that's sad, ken was about to finally confess to her and once again something gets in the way. and it's all cuz rei wanted to tell him that she was gonna start dating tada or something. if ken spoke up first, then it would've changed everything. and couldn't rei take a hint? cuz if she told ken that she's gonna start dating tada and then after that he acts all stubborn and depressed, then it should be pretty obvious that he's acting that way cuz of what she said. and it leads to him liking her.
and in the ending, it was a sad thing to hear. "after that day, rei never called me ken-zou ever again." cuz she's been calling him that ever since they were kids and stuff. so it's kinda like telling him that their friendship is over.