The extrovert meets her match, and the introvert meets herself.
Season 2, Episode 18: "A Friend in Deed"
I've talked before about how the character of Pinkie Pie is often little more than a gag-delivery system and an agent of chaos, and is even used within the narrative itself as a blunt weapon of annoyance, all of which can make it difficult to get a grasp on her as a character. It can also sometimes make it hard to sympathize with her, not just because she makes bad decisions -- they all do, even/especially the pragmatic Twilight Sparkle -- but because Pinkie tends to show little regard for the personal space of others.
For example, this new, more-grizzled donkey that's come into town.He clearly wants nothing to do with Pinkie, but she has other plans, starting with deciphering the initials C.D.D. on the back of his cart. So that last initial stands for Donkey, but what about that first initial? As she goes down the list, he gets in his first of many eye-twitches. If only to get her to shut up, he reveals that his first name is Cranky, and his middle name is ... um ... Doodle. Which it is not okay for her to call him. As he gaits away in a slow huff, we go inside Pinkie's perplexed brain as she tries to figure out why she's not Cranky's new best friend.
Pinkie is determined to make her own personal Frank Grimes smile and be her friend, in spite of the fact that he just wants to be left alone. To that end, she goes through his stuff ...... and eventually destroys his hairpiece, bringing Cranky's baldness to the town's attention in the most overtly humiliating way possible. It's not intentional malice so much as Pinkie's lack of empathy. She cares about others and wants them to be happy because it makes her happy, but she also finds it well-nigh impossible to put herself in their place.
Realizing only after the fact that public shaming is not the way to become Cranky's friend and get him to smile -- a mission she's on against his oft-stated will -- Pinkie gets him a new, more stylish toupee.He warms to her, a little, but would still prefer to be left alone. Which she does not, because what Cranky wants is not important. Pinkie is a sensualist, not an altruist. (Not that I'm suggesting sensualists can't also be altruists, and of course whether or not there really is such a thing as an altruist is a whole 'nother discussion.)
Pinkie continues going through Cranky's stuff as he unpacks, including a snow globe (inside of which is a stealth Derpywatch) from Manehattan and an ornament from Fillydelphia.Cranky's been all around Equestria, and hints that he was perhaps trying to find an old friend of the female persuasion. Pinkie starts to piece it all together when she goes through a scrapbook of Cranky's ... ... which she accidentally sets on fire. Cranky finally snaps and tells Pinkie what's what. Pinkie retreats to Twilight Sparkle's library to mope, where Rainbow Dash is reading a Daring Do book (continuity!), and Twilight tells Pinkie what she needs to hear: Cranky is going to be the exception to Pinkie's obsessive need to be friends with everypony. Pinkie accepts this, provided Cranky will at least accept her apology. But he won't. As he tells Pinkie through his locked door, when Pinkie destroyed Cranky's scrapbook, Pinkie destroyed Cranky's last connection to her. Dingdingding! Pinkie figures it out. The "her" in question is Matilda, an elderly donkey seen at the beginning of the episode, and one who has the same Grand Galloping Gala memorabilia in her scrapbook as Cranky has (had) in his. They met at the Grand Galloping Gala (continuity!) back in the silver nitrate days, fell in love, and never saw each other again. Until now.
The lost lovers are reunited, and more importantly, Pinkie got what she wanted: Cranky smiling and calling Pinkie his friend, which leads to a major endorphin rush for Pinkie. In the Shoehorn, Pinkie relates that while making your friends smile is awesome, it's okay if they just want to be left alone too. But will she remember that by next season? Oh, I have my doubts.