Pinkie Pie, I think, is a childlike egoist. If you leave out a can of treats, she's most likely to just eat them all. She can share fine, but if you don't quickly call her attention on that, she'd just forget about sharing it.
But, I think it's wrong to say that she's an egotist through and through. She has a strong system of things where others always go first. She cracks jokes and throws parties to make others happy. Making other ponies smile is very important to her. And if she feels she really failed in doing that, she would go at great lengths to even get rid of herself so the other can smile again, if that's what it takes. That's kind of what you get in episodes like Too Many Pinkie Pies and Pinkie Pride.
Heck, even in most of her "worst" moments, I think her fault is to ive too much, instead of too little.