Kate and Humphrey's gradual development into a couple, complete with their shared howling session on the train. And nothing more has to be said about the ending.
As Garth tries to teach her to hunt, Lilly ends up flying over their practice targets and makes a fool of herself. Garth is amused, but Lilly tries to hide away in shame. He simply removes the hollow log she got stuck in and brushes away her Peek-a-Bangs to help improve her sight. Garth is wowed by how pretty her eyes really are, much to Lilly's surprise and delight. Shortly after this, Lilly keeps her bangs apart so her whole face can be seen.
The two packs howling after Kate's Disney Death, putting aside their differences to mourn her. Especially Tony who had otherwise been an antagonistic character in the film.
In the sequel, Princess' maternal instincts surfacing as she cares for Runt during his time as the Rogues' hostage. There's something especially sweet when Runt sleeps next to her, showing a level of trust that even Princess didn't anticipate.
As corny as it can be, the ending of A Howl-iday Adventure where Kate, Humphrey, and their cubs sleep peacefully under a Christmas tree at the gas station is heartwarming. Even morseo that the owner, who tried to shoot Kate and Humphrey in the previous film, is the one who left them food and shelter for the night.