Twilight Sparkle is the princess of friendship. More than anypony else, she knows she cannot do anything without her friends. She also knows you are supposed to know a little bit about your friends to have the right to call them your friends.

So when she realizes she knows about nothing about her friends and especially Rarity, the regal alicorn freaks out.

This isn't a comedy, this isn't an adventure: it's simply skimming over a part of the concept of friendship.