Twilight hasn't been living in the tree for very long. She's still trying to figure out how to be both a good friend and a decent librarian. So if there's something going on where the local pegasi have to stay in the air as much as possible and Rainbow just needs her to bring a book up to the balcony, where everypony can see Twilight turn it over... that's really not so much to ask, is it?
A minor act done for one mare is a friendly favor.
Two is a trend.
When you reach the third, herd mentality is already out of the starting gate and galloping.
Four through fifty inclusive is 'Welcome to Ponyville'.
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