I still play because I learn something new about it every time I play it.
When I first played R/B/Y I was just a little kid who didn't know how to play the darn game and eventually found the Missingo Trick and cheated my way up. I still enjoyed it, but let's face it, I was an idiot about how it all worked.
G/S/C made me learn about basic strategy, soon developing a team ( although it consisted of two legendaries but..hey I was 10 so sue me)
R/S/E taught me how well moves work together, what pokemon were good for what and my strategy evolved to an Advanced-Basic level. Plus this is where my love of specific types came into play instead of just the most powerful (Also Coliseum did that too, as it made me love Bug pokemon for life)
D/P/Pt opened me up to more advanced ideas, strategies, and semi-competitive play. Now I understand Game Mechanics well, from Ev's, Iv's, what Nature's do and many other things. And now since I realize that there's a huge competitive field out here, it'll just make me want to keep playing it to see what changes...although I'm not any good at battles (still feels like a noob >.<)
I think I'll be playing this until I'm old and develop arthritis, and even then I'll be playing with my teeth or something