Koibito ga Ubu Sugite Komaru

Hasegawa, a quiet high school student who had been suddenly confessed to by Hiyama, the class' outgoing and cheerful guy. When Hasegawa asked, "What do you do when you're dating?" Hiyama's proposal was a diary exchange. Despite the unexpected innocence, Hasegawa accepted the confession. Two classmates who had never spoken in class gradually closed the gap and discovered love—a pure and innocent youth story!
(Source: Kadokawa, translated)
Note: Chapter count includes 2 extras,