Been into it ever since I was a kid. I pretty much raised myself online, and the communities I was in were all centered around Pokemon (mainly SPPf and here). Since I've got Asperger's, I have special interests, and Pokemon was my first one. Learning all the different ways you could play (normally as well as challenges like Nuzlocke, etc.) and learning that you could play it like a kid (just kind of choosing whatever attack looks coolest regardless of advantage, and things like that) and as you age end up playing it differently and approaching things from different angles throughout your life was fascinating to me. I still go back to things like Gen 2 and find different ways to play it as I get older, or derive different meanings from the same words NPCs might say.
I also really liked the glitches in RGBY and GSC (and RSE, to a point). I've always been into really screwed-up things like that that are, to be honest, often fridge horror, so as a kid and now as an adult that really hooked me.
I think in the end it can kind of be simplified down to the fact that Pokemon's always been with me. It's as inherent to me as, say, my sight, or having opposable thumbs. I'm so into it because there hasn't been a time when I haven't been into it (since the age of sentience, anyway), so a life without Pokemon is simply something I just can't envision.
Also, re: the anime, I do have to say I really like that no matter what Pokemon you're into, you're almost guaranteed for there to be a side character that's into it just as much. While most of the NPCs in the show are never seen again after their episode where they help Ash and friends, it's still cool to be, like, a Shedinja fan, and knowing most of your friends probably hate Shedinja because it's a Creepy Ghost Bug, but then seeing a Pokemon trainer in the anime who is really into Shedinja, and how their Shedinja helps out the main gang because it does something only a Shedinja can do, and hearing the trainer talk about why they chose to train a Shedinja ... you get me. There's almost always someone in the anime who shares your love for your favorite Pokemon, and as cheesy and repetitive and dumb as the anime can get, it's still nice to see.