Everypony knows Fluttershy as Fluttershy. She has the softest, sweetest, meekest little squeals of fearful shock whenever a leaf touches her, she often doesn't leave her property for days, and her animals hear her voice far more often than her pony friends do. Her friends don't really mind her quirks. After all, they all have their differences; it's part of why they work so well together!
But Fluttershy's "shyness" isn't just a character trait. She has something to tell them, and although mental health isn't a topic that's ever really come up amongst her closest friends, she's quite literally saved all of Equestria with them. Several times. Surely that must mean that they have a strong enough bond for her worst-case fantasies to be absolutely untrue and disproportionately bad?