Both for Shizuku and the audience when they realize that the reason Seiji had borrowed all the fantasy books is because he liked her. Especially since both Shizuku and the audience first notice him as rude, only to be revealed as his awkward way of dealing with his crush.
The impromptu "Country Road" ensemble in the Violin Workshop, made all the moreso when you realize that Seiji must have really practiced "Country Road" in order to play it so well, probably in the hope that he would play it for Shizuku some day.
Seiji proposing to Shizuku at the end, which also qualifies for Tear Jerker for the sentimentality.
The end credit sequence contains a brilliant little pantomime between Harada and Sugimura meeting on a bridge, a wordless Meet Cute that perfectly captures the start of budding relationship that takes all of 45 seconds.