"Ah, hello Twilight," Luna said, perking up a tiny bit. "How hath, thou been?"
"I've been good." Twilight responded. "But I'm not here about me, I'm here about you," Twilight said, thinking about how Rarity would do it. "I'm here to... get you outa' the dumps. So... what do want to do?" Twilight asked. Luna smiled a sad little smile.
"Thank you, Twilight. But it will be quite difficult to lift my sister and I from... 'the dumps'." Luna replied. "But I am not against you trying." Twilight nodded.
"Have you ever been bowling?" She asked, as if undaunted by Luna's statement. Luna shook her head "no." Twilight frowned.
"Well, bowling is a pretty hard sport to pick up..." Twilight began to trail off. "So, maybe we could go... flying? No no," She muttered to herself. She didn't even notice Luna had stopped listening and was looking out the window. Twilight was now lost to her thoughts and was pacing around. She eventually paced out of the room, and down the hall. Luna didn't try to stop her, she wanted to be alone anyway.
Twilight's thoughts had followed a rabbit trail and were now focused on "Why are they sad?" rather than, "How can I stop them being sad?" The trail was too long and complicated to explain. She eventually paced all the way to the Canterlot archive. Then, she had an idea.
'Maybe if I can find out what's causing them to be sad... I can fix it!' Twilight thought to herself.
"UGH! I can't find anything!" Twilight groaned. She had been searching for hours and found nothing. She checked her saddle bag to see if she brought a book that would help. What she found, was the journal of the royal pony sisters, which she had procured in (the events known as "castle-mane-ia") a trip to the everfree forest. She decided to give it a read to see if she could find the reason for the princesses despair. She then noticed something about the book.
"Why are there a few pages ripped out?" Twilight asked herself. The last page had a scrap left on it. She began reading aloud:
Me and Luna miss Harmony and Amy dearly. It has nearly been a month now since Amy passed, revealing to me one of the prices of immortality. Starswirl (in his old age) was not able to prepare the everlasting youth spell in time. I have begged him to finish the spell and cast it on himself, but he wishes his life to have a natural end, for it was his destiny. He implores me not to weep, for my destiny is to rule as queen one day. I also fear mother may not recover from planting the tree.
WHY IS THE WORLD SO CRUEL!?!
Twilight looked away from the book, slamming it shut. She couldn't bare to look at it. Tears began to cover her eyes. She looked back at the book:
"Tree? As in, tree of harmony? Maybe there's more." Twilight looked at the book, and flipped past the page she read.
Starswirl's journal has a spell that could bring back Harmony, and possibly Amy, but it requires the elements to cast, and with Luna gone...
Twilight stopped reading, she could probably bring this harmony back! So, Twilight went off (after stealing one unicorn's note book) to gather the ingredients for the spell.