I want to interject into this a few additions.
1) High school-aged Sensei dates Niki.
2) High school-aged Sensei enters the military.
3) He gets injured, and meets Ami's mother while in the hospital.
4) He begins an affair with Ami's mother.
5) He vanishes on Niki, as he believes she has forgotten about him due to his time away.
6) Some time later, Noriko finds him and starts getting tutored by him
7) Soon after that, Maya also starts getting tutored by him
8) A serious relationship begins to form between Maya and Sensei, beyond what he and Niki had.
9) Ami's parents die
10) Sensei becomes a teacher
11) Something causes Kumon-Mi to become isolated from the rest of Japan and the cycle of resets begins.