Keep Out of the Everfree Forest

Sapphire told Emerald her story, which took up the rest of the trip. However, she didn’t tell her the full story.
Sapphire was only in her first few years of school when her parents left for a camping trip and never returned. Since her parents were presumed to be dead, she had to live with her aunt and uncle for most of her life. That’s the part of the story Sapphire did not mention.
What Emerald did hear was that Sapphire “inexplicably” fell into a state of depression and she couldn’t find a way to break out of it.
Eventually, she found a way to help her depression. She found that she could make herself happy by making other ponies happy. During a visit to Ponyville, she received the ‘Prize Pony of Ponyville’ award. She was only visiting for three days!
Her cutie mark appeared there and then. However, it took her two whole weeks to figure out exactly what the yellow smiling face on her flank promised for her in the future. She finally figured out that her talent was simply making everypony smile. Mayor Mare had recently hired her to help Pinkie Pie keep the Ponyville citizens happy.
And that’s why Sapphire was headed to Ponyville. By the time she had finished telling the story, the train had arrived at its destination.