Representing Honesty would make you a lousy liar, I'd imagine.
Also, this was posted elsewhere, and it seems to explain PP's breakdown pretty well:
This latest episode did it for me...
when Pinkie's hair deflated and the whole "pinkie is batshit insane" scene played out, it really resonated with me, mainly because for most of my young life, until high school, I had no friends. I had problems communicating and often retreated into a place, not too different, from the one Pinkie had sunk to. I never did talk to inanimate objects, but I did often make up "characters" and played out many scenarios, from day to day interactions, to drama resulting from conflicting personalities, all in order to fill the void. This only led to further ostracism, many kids teased me and only made it harder for me to effectively communicate with my peers. Thus, I spent more time in my own world.
When I did make friends in high school, I filled a role similar to Pinkie's, that of the "spaz". As I found it the easiest way to make people smile. I was always happy, when I was near my friends and slightly crazy, in the "oatmeal, you crazy" sense. However, I, like pinkie, developed severe attachment issues. Whenever my friends would reject my "spaz kid" shit, I would be quick to assume that they hated me, or that I had no "real" friends. It led to severe bouts of depression and many times I found myself distancing my self from reality to counter it.
The fact is, when Pinkie assumed that she was being rejected and started to talk to imaginary friends , I wasn't thinking "Holy shit! Pinkie Pie is a lunatic who will kill Rainbow Dash and make her into cupcakes", I was crying.