Luna would never admit this to anyone but out of all the citizens in Equestria that weren’t her sister, Twilight was probably her favorite.
It might be wrong to play favorites but given all Twilight had done for her, she thought it was fair. Not that she’d tell anypony but Celestia of course.
Twilight was one of the bearers of the Elements of Harmony which freed her from the darkness known as Nightmare Moon, at the time she just found her one of her six saviors but that changed when she returned to Ponyville for Nightmare Night.
Twilight was one of the only ponies who were willing to help her adjust to Ponyville, while everyone else either ran in fear or just found it fun to pretend being scared. Twilight was there to help her when no one else would, which Luna was grateful for.
She was also grateful for the time when Twilight was able to help her forgive herself for her actions as Nightmare Moon. Yes, it was to banish the Tantabus, but practically everything Twilight had said to Luna that night had come out from the heart, she knew Twilight wasn’t just saying that so Luna would suck it up. She was genuinely serious about forgiving Luna for her actions as Nightmare Moon and cared enough about her so that she’d stop harming herself with constant nightmares.
That’s what she liked about Twilight Sparkle, she was a kind individual who prioritized most ponies’ needs over her own, provided that they aren’t someone like Tirek or Chrysalis of course.
Luna had to admit, she was worried about the constant invasions Equestria faced, especially when Twilight would be targeted. Heck at one point, she spoke about it with Starlight in one of the unicorn’s nightmares after the whole King Sombra ordeal. Basically Luna was just glad she stopped the nightmare in time.
At that point Luna knew that they had to take action in defending Canterlot so Twilight would be safe at such a time when an enemy attacked, and so her friends would not suffer the pain of losing her.
That’s why she got into a disagreement with Celestia over letting Shining Armor plan the defenses. She liked Shining Armor just fine but given how easily manipulated he was by Chrysalis when he was supposed to defend Canterlot, she wasn’t sure he was the most reliable source of help.
Luna definitely regretted not being there for the Changeling invasion, even if at that point she was supposed to be resting, she still felt bad she couldn’t be there to help Twilight in the time she needed her help.
In the end, Luna wasn’t surprised when Celestia had decided to give Twilight the title of ruling Equestria. She was clearly capable of doing so given how many challenges she and her friends faced. Twilight was after all a very organized and courageous leader when the situation required it. Add that to her kind and helpful nature, that’s what Luna appreciated Twilight for.