Regarding recent events... (S3E13 spoilers) (No, seriously, if you somehow haven't heard of all the stuff, don't read this) · 6:46pm Feb 1st, 2013
Two things - one, my friend's theory, the other, my own thoughts.
Firstly, my friend's idea:
"I mean, according to McCarthy, the Princesses are basically just really good people. This is the next step of Twilight's training - sharing what she's learned with others, Think about it - there have been no friendship reports this season. Celestia must then think that Twilight has learned enough about friendship that further lessons would be redundant."