Myke Greywolf said:
I think Luna was genuinely depressed, but not about to turn back to being Nightmare Moon. She was just sad, and as Twilight approached and called her, Luna decided she didn't want Twilight to see her like that, and reverted to her previous, regal façade. Her looking at Nightmare Moon's statue was just done as a pretext for being at that place that symbolized the image everypony had of her.
Luna only started opening her heart once she saw who she was speaking to, and what were Twilight's intentions.
Nah, just check it out in pictures.
Nightmare Moon "You belong to me!"
Luna: "Yes, mistress, here, I brought you some candy."
Interruption shock
Angry defiant look on face, hoof pulled back
On an obvious level, it's about Luna interacting with Twilight, but on a subtle level, it's about Luna interacting with Nightmare Moon.
karby said:
The reason I think her happiness is unclear is because Luna Mays is yelling, thereby giving off the impression that she is anything but happy. But it's just her royal Canterlot voice.
Actually, that's not it either.
Luna used the Royal voice when she first appeared in town, but she forgot to use it once ponies started running away from her. Then (even after the scene pictured above) she chatted pleasantly with Twilight about her costume.
Then she talked like possessed-Galadriel when speaking about her former power, and then when talking about her feelings, she became merely hostile and confrontational. And she blamed it all on the Royal voice.
It's like losing control and yelling at someone, and then denying it, saying "That wasn't yelling."
Twilight believed her, which was a good way to get off-subject and move back to something constructive.