How the Toads react when Luigi frees them is always pleasant, especially the first Toad, who hugs Luigi and calls him his hero. Luigi even gives the little guy a pat on the head.
The. Entire. Ending. Full stop. Why? For many reasons:
Luigi 's reaction to having saved Mario. At first, Mario just pops out and gratefully thanks Luigi for it, to which Luigi replies with a shy, hesitant "I-I did it?". Shortly afterward, as they watch the Dark Moon return to the sky, Mario gives Luigi a pat on the back, giving us a proud "That's my bro!" To which Luigi replies with a quiet "Th-Thanks, M-Mario...!". Luigi is Adorkable incarnate and you will love every second of it.
"Ha ha! Ah, Luigi! You saved-a Mario! Way to go! Number 1!"
A Greenie takes a picture of Luigi, Gadd, Mario, the Toads, and some ghosts. Luigi is in the center and Mario is off to the side. He's had his ultimate shining moment, and he hangs the photo on his wall.
Two words: "Here, Doggy!". Not only has Luigi learned to befriend ghosts, he not only forgives Polterpup for messing with him before, he adopts him. That's right, Luigi adopts a ghost—and they look just absolutely happy together.
Finally, as of this game's end, it seems Luigi has finally—if only mostly—kicked his fear of ghosts. Luigi has not only proved himself a true hero from Luigi's Mansion until now, but overcame his own phobia by facing it head-on through sheer willpower to save his brother first, and the ghosts that King Boo drove amok this time. This is not just heartwarming in the development of his character, but doubles as an incredible Crowning Moment Of Awesome.
All of the ghosts are so cheerful after the Dark Moon is restored. Among the ghosts shown changing back is a Slammer chewing on a Greenie's head. After regaining its senses, it realises what it was doing and spits out the Greenie and proceeds to cuddle it.
Just seeing Luigi, Mario, and E. Gadd playing with the once again friendly ghosts is Sweet Dreams Fuel by itself.
Before one of the last missions, E. Gadd tells Luigi that if he doesn't come back, he should know just how proud of him he is.
Luigi's reaction to getting a 3 star rank. He looks like he can't believe his eyes, then he does a little dance, hops, and claps his hands, all with a big smile on his face. Seeing him feeling proud of himself always feels good.
After you recover the water system in the Haunted Towers, the water starts flowing everywhere as massive currents, and you can see Greenies hopping in and playing, as happy as they can be.