Other Foals Used to Tease Me

 "Stop horsing around, Fluttershy! We've got a lot of training to do. Come on, now. what's going on?"

"Well, y'see. Hhuh. Well..."

"Oh, Rainbow Dash, I just can't do it! I can't fly!"

"What're you talking about? Just last week you went into that wicked nosedive to save that falling baby bird right before it hit the ground!"

 "But that was different; that was an emergency! This whole tornado thing -- it's more like a performance, and you know how I hate performing in front of others! Don't you remember flight camp? I couldn't gallop hard or fly fast..."

 "...Not with everypony looking at me..."

"It wasn't that bad."

 "You're right, Rainbow Dash... It wasn't bad... it was horrible."

 "All the other foals used to tease me... a lot!"

 "Fluttershy, Fluttershy, Fluttershy can hardly fly! Fluttershy, Fluttershy--"

"Ohh... I just can't risk that sort of humiliation again!"

Episode: Hurricane Fluttershy

My commentary: One of the absolute best things about Fluttershy is how relatable she is, so seeing her reflect on how she was unpopular and uncomfortable among other kids growing up just adds to this. It's also nice to see her stage-fright noted, as some episodes seem to ignore it. I like her hesitation to explain it all to Rainbow Dash, as it's a very realistic moment.