Responses
I think they dated before
i think he went to military service since there is 2 years difference
Oh, I completely forgot about the military. So maybe something happened before he went to military? They broke up because of some misunderstanding or mistake and started avoiding eachother after he returned.
Either the third pair have been a couple in past and broke up due to a serious incident (the reason why they are in different years despite being of same age) or they were friends who had crush on eachother and drifted apart due to the same incident.