Unmarked spoilers are ahead, per Spoilers Off rule.

Season 1

In Episode 6, the make-up scene with Nowaki and Hiroki in the library. Counts as a Tear Jerker too, if you think about why Nowaki left Hiroki in the first place.
- Later, Hiroki finds the heartwarming letters that Nowaki planned to sent but didn't want to because he want to quickly finish overseas education and return to him. A pretty sweet moment considering his Death Seeker Status in the early story.
Hiroki:I think that I can live to 150 years with these. *Hugging Nowaki's letters while Nowaki hugs him*

Miyagi telling Shinobu that their age difference means nothing when they're 117 and 100.

The Egoist story from Junjou Mix has Nowaki asks Hiroki if he would accept the idea of buying rings as proof of them being at each other's side. Hiroki saying yes also makes it much sweeter.
- They also got the rings on Hiroki's birthday.
- Prior to the Mix series, in a Christmas Chapter, Hiroki did contemplate on the idea of buying couple rings but decided to buy a more practical Christmas gift for Nowaki. It seems that Nowaki wasn't the only one thinking about this.