When I first got into Pokemon in 1999, I used to be interested in it outside of the main games, like the spin-offs, anime, trading card game, etc. Shortly into Gen II, I began to lose interest. By Gen III, I had lost interest in everything aside from the main games. Even still, I didn't play a main game (some replaying aside) in between Ruby in 2003 and Diamond in 2008. Probably largely because I didn't like Gen III at all. My interest in main games came back, though. The interest in the spin-offs, anime, TCG, never came back, though.
The only other thing that comes to me off the top of my head was instruments - in the early 00s, I though the trumpet was my instrument. In 2004/early 2005 I had shifted to thinking that clarinet was my one true instrument. While I've switched instruments after that, I've remained pretty uniform in still loving the ones I switched from. Trumpet I was very happy to switch from, and my love for the clarinet didn't last long (bass clarinet aside!).