@Mnuchinboo
Honestly, i don't know either. I think it's because Takamine was hoping that by being friends with him, he'd be able to let go of his height insecurity. Yuuki kept bringing up his pacing as slow, so to not make Takamine worry(?) he apologises for it and even tells her to leave him behind when it rained. I think he started to feel that Takamine was starting to not feel happy being with him, since she was being stiff and not talking that much, as well as wanting to go their separate paths, furthering his insecurities. However, when Yuuki stated he was happy with being with her and thanked her, and he saw her smiling, he started to fall in love. Which leads to this chapter, where his insecurities about love start to resurface after the "smiling" makes him realize he loves her. However, as much as he tries to hide it, somehow Takamine can sense somethings wrong.
To surmise, i think Yuuki, up until the "Smiling" was somewhat using Takamine as a bandage/therapist for what had happened at the confession. Honestly though, i'm just grabbing at straws.