Well, let's review.
Why did I buy Yellow, some ten years ago? Curiousity. Everyone seemed to like Pokemon, so I decided to try it. Turns out that if you can get past all the merchandising and the hideous anime, there's a decent little game in there.
Why did I buy Diamond, about two years ago? Social pressure. I joined a pokemon-themed MU* on a whim, and then found myself wondering what everyone was going on about with all this new stuff.
Why did I buy Soul Silver, last year? Several reasons. Diamond was pretty good, but it had some irritating issues that got fixed in the new version. What's more, there was still an entire region I had yet to explore (having bypassed the entire Gold/Silver/Crystal, FireRed/LeafGreen, Ruby/Sapphire/Emerald era).
Why did I buy Sapphire, later last year? Completion. Not only was there still another region I hadn't explored, with an entire generation of Pokemon with which I was unfamiliar, but there were Pokemon I wanted to add to my collection that none of my current games could get.
Why did I buy LeafGreen in October? Nostalgia. Yellow was long-gone to me, but I was very interested in returning to Kanto and basking in my memories of the original 151, especially since Soul Silver had given me a taste of Kanto in the second half of the game.
Why will I be buying White? Innovation. Not only do I want to have some fun with the new Pokemon, but I'm curious about the new gameplay mechanics, I want to try out the new attacks, I want to explore the new area, and I want to see what they're doing with the series to take it forward.
So if you want a single, unifying reason why I keep buying Pokemon games...sorry, I'm going to have to disappoint you. But if you want a single, unifying reason why I buy Pokemon games in the first place?
They're fun.