UPDATE 1: It looks like "Why is she so sad?" is a common question. I would rather you fill in the blanks with your interpretation, my own being that this is a rare moment of insecurity for Applejack.
is fairly close to what I originally thought as I drew this: Sometimes... a girl... just wants to be a girl. "Why don't any pony tell me I look pretty too..." Applejack thought, thinking back to all the attention her other friends seem to get. Even ol' Twilight seems popular and she don't even notice. "I can be pretty if'n I wanted, I just- I just have more 'portant things to do..."
's comment, "You look gorgeous, sweety!" might cheer up the AJ in the above interpretation (and I agree with the sentiment)
(Stef0Pal)'s comment below mentions that AJ looks like her mother, so every time she looks in the mirror... This suggests a deep sadness, and would fit the image well.
's comment actually gave me some anxiety as I read it, going through a list of worries that AJ has to deal with on a daily basis as well as some close calls.
So, like I said, I prefer it when you fill in the blanks as it gives any number of interesting interpretations.. ---
UPDATE 2 's short story, Stetson, pairs up with this pic. Warning, you may shed a tear or two.
It looks like she is seeing that nobody sees her as a good pony or anything like that. I totally wish I could say "Applejack, you are absolutely beautiful. Don't you ever let anypony tell you otherwise."
Sorry, but Applejack is by far, the prettiest of them all in my opinion. Not a doubt in my mind about that. If only I could skadoo into their world and let her know how I feel about her....
Not a doubt in my mind about that.
If only I could skadoo into their world and let her know how I feel about her....