This might just be completely incorrect when the next chapter comes out but I think what happened was:
MC confessed his love to CF (childhood friend), CF even though also loves him, for some reason didn't accept (probably because she was too shy or smt), MC probably thought that CF didn't like him and then said something along the line of "my feeling will never change, I will be next to you and wait until you accept me." or something and then she probably just nodded her head instead of saying something.
So over the next few years leading to the manga begining, MC still thinks that CF doesn't like him and but sticking around cause he believes that the nod she gave in the past might give him a chance. Meanwhile, CF now having the "my feeling will never change" insurance, and has gather some courage (probably by becoming the smart, beautiful, angelic girl that everyone in school loves) also just wait around for MC to confess again, not realizing that to MC, his confession has already been said and he is just waiting for her answer.
So basically, MC waits for to CF gives her answer for the confession he gave in the past, while CF waits for MC to give her a new confession, and neither of them know what the other thinks or tell the other anything.
And then the manga started.
This is my theory, this might be completely bullshits, but it seems plausible with the infomations we have been given, plausible to me at least.