Sweetie Belle is curious, just like any little filly her age should. Just that she just had to ask, and Rarity is the natural first Pony to go to. What the response and the answers will be, is why she asked in the first place.
I just recently started to write stories directly towards the FiM actively, though I have been writing for years, publishing numerous stories at Mibba and the eventual pony story, as far as to the MLP