The Real Reason Anime Theme Songs Change So Often
Have you ever watched an anime and fallen in love with its opening sequence only to be disappointed when it changes just a few episodes later? It's common practice for anime series to switch up their opening theme songs and animations fairly frequently. Most of the time they don't even last longer than a single, solitary season, and it can be a huge bummer to see your new favorite song get left behind so quickly.
But why would anime studios do this? Oftentimes the first song and sequence are incredibly entertaining on their own. A switch such as this isn't really necessary, and there isn't always an immediately apparent reason behind them. However, it's certainly a purposeful decision on the part of the anime's creators. Luckily, it doesn't take too much digging to learn that even though anime studios don't have to switch openings, they do so for a clear artistic reason.
Different anime openings denote different seasons
Broadly speaking, anime are special in that their opening themes do a lot more work than the sequences used in American shows (which often last for a show's entire lifetime). Both are supposed to give viewers a feel for what they're about to see, but anime sets itself apart by including longer, elaborately animated sequences that often foreshadow crucial plot points or suggest more details regarding the characters and world. When an anime changes openings, it's often to reflect similar changes in these elements.
Just take a look at one of the world's most popular anime, Jojo's Bizarre Adventure, which is famous for its mix of unique opening themes and borrowed ending themes from Western pop music. JJBA is usually divided into distinct parts, and each part has a new setting, new characters, and a new tone. As a result, the animation studio changes the opening to reflect the new story.
Usually, this happens between individual seasons, but not necessarily. Take Part 3 of JJBA, Stardust Crusaders for example. For the bulk of Part 3, the anime opened each episode with the song "Stand Proud" and its associated animation. But, at Part 3's halfway point (which coincided with the heroes nearing their journey's endpoint), they switched to the song "Sono Chi No Kioku ~End of the World~." So, anime studios generally alter the opening to reflect the story and themes of the anime at the time.