Twilight didn't know what her life would be like without her friends.
She had known them for so long that old Twilight who lived in the library as a recluse seemed like a faded memory.
Together there was nothing they hadn't faced. Nightmare moon, Discord, Changelings, Tirek, and heck even the occasional dispute amongst them.

The hardest challenge so far? Twilight could easily chalk it up to having faced Starlight Glimmer. She had so many friends who would do just about anything for her, but the friendship had been one sided. Starlight Glimmer loved only herself, which resulted in her being lost inside the caves.

Despite this Twilight still had her friends and her role to spread that friendship across Equestria. What more could anyone ask for?
Yet, something was amiss... Twilight couldn't put her hoof on it, but she could've swore the last time anyone had seen Starlight Glimmer was when she vanished into the caves. But here she was, appearing in the corner of her vision at a conference. It was unnerving, and Twilight aimed to get to the bottom of the mystery.

(My take on Season five's finale. This is canon divergent obviously.)