The MC says in his mind that he has that personality because of the people that surround him is talented or overshadows him that he can never have any kind of pride and such (what do you think would happen if surrounded by such people. It's just sucking up all the postive emotion of joy and such from him.). So, he can never smile with any kind of postive emotions (it's better explained in the story) which the two girls are trying to melt that "frozen heart". Kirazaka Rei has a past with the MC (not just only liking the MC because of the way he acts) but not truly revealed yet, however I can guess that the MC said something to her that nobody has to her because of her looks and such. Kanzaki Shizuku has spent her childhood with the MC but never seen him truly smile (love is confusing being with someone for a long time can develop feelings for them and her childhood past has yet to be revealed at the current chapter that I'm at). I have the same thoughts as the MC; what is a relationship, but I don't need it. He doesn't understand it and I wouldn't say he's dense but lost and confused about the postive emotions that others feel but he can't feel them. It gave me the goosebumps when reading the part that he never smiles in every picture that he has been in and it's hinted in various parts of the story. The MC's outside is what you call ordinary but the inside is nothing alike the outside. His thoughts are thoughts that I wouldn't come up with and I'm impressed at the author of think about such thoughts, like "Preference of a man/woman is just a thought that isn't always true, even you say that you like someone because of their personality, it's not always true since people judge and talk about other for their looks." I don't completely remember what he said but it's close, however this doesn't even scratch the MC's mind. I can go on but I'll stop here and read more to have more information about this MC. Btw, the MC knows that the two girls like him but he can't answer them because of the "wall" of emotions that he has to break.