Immediately after her ascension, Twilight's feeling stressed. Princesshood? Wings? It's a lot to take in, and the full weight of it is only just now sinking in. No matter how much of an honor it is, Twilight can't stop panicking over it.

So when Celestia gives her a choice between being an alicorn and being a unicorn, Twilight's not sure which path she wants to take.