"Please, Twilight," Luna pleaded. "You are a princess of Equestria now. Please call me Luna."
Twilight rolled the idea over in her mind. She wanted to comply, she really did, but after thinking about it, she decided that she couldn't. "I'm sorry Princess," she said, her ears drooping as she looked down at the ground. "I just... can't."
"But, why?" Luna asked, puzzled.
"Well..." Twilight paused, looking back up, but averting her eyes, and wondering how she was going to put it. "I know I'm a princess now, and I know we're supposed to be equals, but it just doesn't feel right. You know? I mean, you're the Princess of the Night, and I feel like I'm still just a glorified librarian with a horn and wings."
Luna cocked an eye at this, but Twilight was looking back at the ground again, so she didn't notice, and continued.
"You've got purpose. You raise the moon and control the night, you rule Equestria and have fought battles along side Princess Celestia, and you're far older and wiser than I could even comprehend at this point." Twilight cringed inwardly as she realized she'd just called Luna old, but hoped the part about being wise would cover for it, and decided it was best to just move on.
"As Rainbow Dash would say, you're awesome." Twilight said with a small smile as she glanced towards Luna, hoping to give things a positive spin. Unfortunately, she couldn't read the expression on Luna's face, and her smile quickly faded as she looked back down. "I guess what I'm saying is, I just don't feel worthy to call you by such a familiar name. Beyond that, we really barely even know each other. Maybe I will some day, if we're able to spend more time together and can really call each other friends, but for now I just can't. I'm sorry."
Twilight tried to look Luna in the eye as she apologized, but found that now Luna was the one staring at the ground with tears beginning to form in the corners of her eyes. "Oh, no! What have I done?" Twilight thought in horror. "I've offended her! What do I do now?"
Twilight didn't get the chance to think about it for long, though, as Luna then spoke.
"I understand your feelings," she said softly. "But please remember," she continued, as her voice broke. "It was you and your friends who saved me! If ever there were a pony who was unworthy of another, it is I." Luna's tears were now uncontrollable, and rolled down her face as she paused for a moment to fight back sobs.
She looked back up at Twilight, who was now torn between terror and remorse. "So please, Princess Twilight Sparkle," Luna said, getting down on her knees in the deepest bow she could manage, her nose pressed against the cold, stone floor. "What shall it be? May I call you my friend?"
Now it was Twilight's turn for tears as she viewed the prostrate Princess of the Night before her. How could she possibly say no? "I... I think I'd like that... Luna," she said, offering Luna a hoof up.
Luna accepted it gladly and stood once again, the two ponies now looking at each other through tear-filled eyes. "As would I, Twilight," she said as they embraced.