You are My brightest star

During a rainy day, Samon stopped at a flower shop to take shelter from the storm. There, he met Koito, the girl from that store. After that, when he finds himself involved in the love affairs of his classmates and finds himself alone in his class, Samon projects his fury on Koito saying "What amuses you so much so that you are always smiling?" and ends up hating himself. That made Samon anxious, and on graduation day, for some reason, Koito gives him a diary. Actually, Koito admired Samon. In there, one line caught his eye, "Life during middle school was fun because you were there..." Despite hating himself, Samon likes the things written about him in Koito's diary. Time passes and upon entering the second year of high school, he reunites with Koito, who enrolls in the same school... (Source: LemonadeScans, edited)