"Party of One" is a popular MLP episode because it showcases the character development of Pinkie Pie. In this episode, Pinkie Pie becomes increasingly paranoid and suspicious when her friends start canceling their plans with her. This leads her to throw a party for herself, where she imagines her friends as inanimate objects and talks to them. The episode explores themes of friendship, trust, and the fear of being left out. It also highlights Pinkie Pie's resilience and ability to find joy in her own company. Overall, "Party of One" is a fan-favorite episode for its humor, emotional depth, and memorable moments.