Pinkie was not the happy pony she pretends to be, nor does she actually enjoy throwing parties or pranking, it's simply all she knows how to do. She was really terribly miserable, a result of her traumatizing home life and the lie she was forced to live. Ever since Rainbow stumbled across her and learned the truth, staying her friend regardless, she felt for the first time in a long time she could be happy.

Sadly, there is another who suffers the same way she does, and it's a pony she can't make happy, no matter how much she may want to or try.

Note: It isn't entirely necessary to read the first one to understand what's going on here, there is enough context and details given to get the gist, but I still highly recommend reading Fake Smiles first.