AJ just keeps getting more amazing. Cmon, you know you loved how she took on that chimera. That episode gave Daring Do a run for her money.
Also I've read some headcanons about on the interwebs regarding Applejack excessively doting behavior towards Applebloom. It makes sense. Think about it, AJ was probably old enough to remember losing her parents. Ever since then she's had to take on the mother role for her sister. She can't bear the thought of losing another loved one too, especially considering the kind of sister-sister & mother-daughter double-whammy of a relationship these two share.
Another thing I really appreciated about this episode is the fact that the moral of the story is specifically intended for the adults watching the show, or any older person in a similar situation to AJ's. At some point, you gotta let em go.