Applejack

Spoilers for an upcoming Applejack episode I'm excited for in here, so I'll hide it under a break.

Given the knowledge that we're going to have an episode about Applejack's fillyhood and lying soon, I'd like to just air out a few theories about Where the Apple Lies.

I'm 99% sure it's going to be set after the death of her parents. I'm not sure if they'll ever address it but I'm hopeful that this is the episode that'll finally clinch it by maybe mentioning them properly.

See, grief is a weird thing, and it especially has some weird effects on children. It's not out of the realm of possibility that Applejack started to lie as a filly because she was reacting to her parents' death. The consequences of this lying, according to the synopsis, is that her family ended up in the hospital.

Now, I don't know about anyone else, but this could add a whole extra layer of Applejack's personality that we've not considered before.

Imagine yourself in her place. Your parents are gone, and you had what some would call a 'bad' reaction to grief. Then, because of this, the rest of your family ends up in the hospital. That stuff is scary to a foal. What if you'd killed your granny and your siblings, too? And it's all your fault. You don't want to lose the family you have left, ever, and this near miss has made that incredibly clear. Even if it turns out they all just broke a leg or caught the 'flu or ate something weird, a child isn't gonna be that rational. They're in the Hospital so this is Serious and really her only probable reference for that is her parents dying.
So she decides to value the truth. To become somepony that her family can depend on instead of be hurt by, and by extension, she also becomes reliable to the other ponies around her.

It explains why Applejack is so bad at lying - every time she does it, she has that reminder in her head. - You are hurting this pony, what if something awful happens because you didn't tell the truth, you don't want it to happen again.
It explains why she was defensive and stubborn in Applebuck Season - "Are you sayin' my mouth is makin' promises my legs can't keep?" - because she's not like that anymore, she doesn't lie. Not only that, but she's now the Bearer of Honesty. It's both a confirmation of all she's been trying to prove to herself, and also yet another thing to live up to.
It explains why she couldn't come back after The Last Roundup - to do so would be admitting that she failed, that she lied about getting the money for the town even if she had no real control over that.
It explains her crazy over-protectiveness of Apple Bloom even more than the whole 'my parents died and I don't want that to happen to my little sister'. It's also got a silent little 'because it nearly did and it was my fault' on the end.

It just, it explains a lot. About her character - about who she was , who she's always struggling and striving to be, and who she is.