As a manga that deals with suicide, its repercussion, cause, and how to prevent it, the series is a walking Tear Jerker by itself. There is nary a chapter that doesn't have something tugging the heartstrings.
All spoilers are left unmarked.
- Kakeru's Stepford Smiler disposition in general, as shown when he casually lies about why he missed out two weeks of school as "slacking off". He's dealing with his mother's suicide that he thinks is his fault.
- Naho's massive My God, What Have I Done? after she learns that asking Kakeru out during the first day at school indirectly caused his mother's suicide.
- Naho's reaction when she learns from the letters that Kakeru will die due to an accident sometime in the following year. Double the reaction when she learns that it's not due to accident, it's suicide.
- The gang tearfully reminiscing about Kakeru and how he never got to write a time capsule letter before they visit his grandmother's residence. There's also their somber regret when his grandmother reveals about his suicide.
- The second half of the Bon Festival chapter, specifically, when Naho and Kakeru are paying respects at a shrine.
- Naho and Kakeru's fight during New Year. Something that Naho has been treading carefully not to do has finally happened. They seldom talk for the next six weeks, a huge loss considering that in retrospect Naho means everything for Kakeru at that time.
- In the original timeline, they never made up. Ever.
- Chapter 19. It's a retelling of the original timeline from Kakeru's perspective and is basically a Trauma Conga Line for him as it reveals bits by bits how he eventually chose to take suicide as an option. The part where his suicide is actually only the first successful attempt is particularly heartbreaking.
- Kakeru finding his mom's final message revealing that his mom committed suicide not because she thought that he didn't care for her anymore, but to spare him from the burden of being with her now that he has friends. It's not so much the content (which is heartwarming), but because it's the final impetus for him to finally commit suicide, at least in the original timeline.
- Suwa (or at least his future self) considers himself proposing, dating, and marrying Naho after Kakeru's death as taking advantage of the situation and describes himself as unfair. Repeat, he considers his own happiness to be a mistake. If you don't believe Suwa to have an inferiority complex, however well-hidden it is, then this is the moment where you should.
- The realization that while there may be multiple timelines where everything can be righted, there'll always be one where something that goes wrong will stay wrong. The new timeline allows Kakeru to survive, yet another - the original one - doesn't allow him to. There's nothing that can be done about it.
- This is best exemplified in the very last moment of the series, where Naho's future self in the original timeline tearfully bids her younger alternate self goodbye after depositing the letters, wishing that she may set right what she did wrong.
