Derek's single minded determination to get Odette back even when everyone else has given up on her. "Everlasting love" indeed.
It's kinda sappy but...
Odette: Will you love me, Derek, until the day I die?
Derek: No. Much longer, Odette. Much longer.
When Derek finds Odette after searching for her for most of the movie. Remember, the last time they saw each other was when Odette turned down Derek (after spending most of their childhood bickering). There is no argument, no harsh words, nothing but Odette simply saying "Hello, Derek" and Derek runs to her as the music swells.
Puffin's promise to repay Odette after she helped him.
Puffin: I owe you, Princess, and I intend on staying until me debt is paid.
A Royal Family Tale
Odette and Derek helping the woodcutter do his job after since his arm is injured.
Just every single interaction between the main characters and newest one, Alise.
Odette tries to get Alise to talk through song, singing that she wants to hear from her.
After finding out that Alise has no other relatives, Derek excitedly exclaims that "I'm going to be a Mom".
Queen Ubertha buying every little thing that she thinks Alise will like, even a pony.
Odette comforting Alise after she has a bad dream.
Alise addressing Odette as "mommy" immediately after the climax and the following hug. It happens again, this time with Derek, in the epilogue.
Princess Tomorrow, Pirate Today
Derek and Odette continue to be Royals Who Actually Do Something by helping some commoners build a bridge. This cuts their travel time from "all day" to "five minutes".
How does the Scullion army find the unity to properly row their boat? They chant "For! Alise! For! Alise!"
The first thing Alise does when she sees Scully's ghostly former is hug him (or try to, anyway). Then she thanks him for saving her in the previous movie.
Lucas revealing himself to the Boggs in order to help Alise escape the island is a big friendship thing but what makes it truly heartwarming is that he's basically exiling himself. He can't live on the island they know he's there because they would hunt him down.
Alise chasing Lucas down, flushing him out of hiding and ultimately tripping and tackling him in order to make sure he isn't sad and alone anymore.
Thanks to Alise and Scully, Luca overhears his parents talking about their real reason for wanting to send him to an orphanage and, incidentally, gushing about what a great son he is.