Thats just who she is. Pinki pie is filled with love and friendship and often finds different ways of expressing it, also she eats a lot of suger filled foods, so that may have something to do with it....
The honest answer- because it's a girl's trait.
Now what I mean by that is that when Lauren Faust (the genius creator of MLP:FiM) was making her characters, she wanted relatable characters. She wanted them to all harmonise together. She did that by giving each character a stereotypical trait of a girl, and what friendship is. After the final changes she came up with:
Fluttershy- the shyness (a trait Lauren had as a kid) & kindness of friendship
Rarity- the feminism- & generosity of friendship
Applejack- the country girl- & honesty of friendship
Rainbow Dash- the competitiveness & loyalty of friendship
Pinkie Pie- the silly girlyness & and laughter you find in friendship
Twilight- the crazy bookworm & and magical spark that makes up friendship.
So everypony got a trait, and personality, making it easy for the viewer to find a character to relate too. I mean, 6 chances you can relate to one of that. That's why the characters are a big reason so many adult males watch the show. Everypony is unique. Pinkie Pie Just happens to be the fun, silly, hyperone.
Pinkie pie's nickname is Pinkie, or sometimes super-duper party pony. She is silly and fun. Sometimes a little awkward.
A pony toy who is pink and 14 yrs old
Pinkie Pie will tell you
It was left deliberately ambiguous if Pinkie Pie is related to the Apples. If she is related, she is only distantly so. This will likely not affect the show in any significant way.
Twilight Sparkle
No. Pinkie Pie is usually quite cheerful.
Then Pinkie Pie will be one year older than Fluttershy
Pinkie pie's nickname is Pinkie, or sometimes super-duper party pony. She is silly and fun. Sometimes a little awkward.
A pony toy who is pink and 14 yrs old
[Pinkie Pie]Ooh! This is my jam!There is a placeWhere the grass is what's for dinnerApplejack: Soup's on, everypony![Pinkie Pie]Charmed, fun, and wildApplejack: Yeehaw![Pinkie Pie]There must be something in the waterSpike: What?[Pinkie Pie]Sippin' rainbow juiceTalking Elements of HarmonyRarity, Twilight, Applejack: Yeah![Pinkie Pie]Our BroniesBraeburn: Hey there![Pinkie Pie]Hang out tooPonies: (laughing)Spike: Come on, Bronies![Pinkie Pie]'Cause they know we're awesome filliesSpike: Come on, everypony![Pinkie Pie]You could travel the world (You could travel the world)But no one can groove like the girls with the hoovesOnce you party with poniesSpike: Party with the ponies[Pinkie Pie]You'll be seeing Rainbooms!O-oh o-oh o-ooh!(Chorus)[Pinkie Pie]Equestria girls, we're kinda magicalSpike: Equestria![Pinkie Pie]Boots on hooves, bikinis on topFurry coats, so cuteWe'll blow your mindAoaoah oh, aoaoaoh!Equestria girls, we're pony-fabulousSpike: Equestria![Pinkie Pie]Fast, fine, fierce, we trot till we dropCutie marks representSpike: Cutie marks![Pinkie Pie]Now put your hooves upSpike: Put yo' hooves in the air[Pinkie Pie]Aoaoah oh, aoaoaoh![Male Backup/Spike]Break it down, DJ Pon-3These are the ponies I love the mostI wish you could all be Equestria girls
No. Pinkie Pie has threes sisters, all of whom are earth ponies.
Pinkie Pie will tell you
think you should pick pinkie pie 'cause shes awesomer and is the best PARTY PONY EVER, i totally lover her so yea!But then again, Rainbow Dash is 20% cooler. Hmm. Tough choice. Just do whatever you like. If you're more of a cool, fast, athletic person, go with Rainbow. If you're more funny, random, crazy, go with Pinkie.
It was left deliberately ambiguous if Pinkie Pie is related to the Apples. If she is related, she is only distantly so. This will likely not affect the show in any significant way.
Twilight Sparkle
Pinkie Pie and Twilight have a close friendship in the show "My Little Pony: Friendship is Magic," and Pinkie Pie is shown to care a lot about Twilight. Their bond is based on trust, understanding, and support for each other throughout the series.
Yes, Pinkie Pie did dress up as a chicken for Nightmare Night. (In Equestria, they have Nightmare Night instead of Halloween.)