There's something rather sweet that Horton the elephant's best friend is a mouse who never stopped trying to help him.
Horton and the Mayor, despite their unusual circumstances, grow to be close friends, giving each other strength when their own worlds didn't believe in them.
Somehow they managed to sneak the line from Horton Hatches The Egg into the movie, as Horton explains that he promised the mayor to get whoville to a safe place. "And an elephant's faithful 100 percent."
The whos finally believe in the mayor, cheering for the elephant that's going to help them.
Rudy giving Horton the clover, and shortly after, Horton giving Kangaroo the cookie.
And then Kangaroo placing a small plant like an umbrella over the clover.
When Horton starts to sing "Can't Fight This Feeling," after all he did to help the Whoes against all odds, no one deserves to called "A Candle in the window/On a cold dark winter's night" than him.
The fact that everyone, including Kangaroo and Vlad, sang along the Sudden Musical Ending, gives the viewers a good feeling that everything finally turned out alright for everyone.
Jojo joining in. After spending the entire film (and implicitly much of his life) saying nothing his voice really has a chance to shine here.