I play for a lot of reasons.
For one thing, definitely the nostalgia. I mean, I have played Pokemon since I was a very young kid. My first Pokemon game was Gold, and when I bought it, I was so addicted to it that for a very long time I ignored all of my other games just to pay attention to my gold. Then I managed to get Silver and Crystal, which totally stole my heart away.
Another thing, Pokemon is addictive as hell. I would like to see someone who actually enjoyed the games just give it up. It's almost like a person and his drugs. He can give them up, but it feels different and most of the time you have cravings and just aren't happy. I don't much care for RPG's, but Pokemon I am more than happy to play.
Another thing is the fact that when I was little, I wanted to create a perfect team and have battles with all kinds of people. I couldn't due to the whole Game Link Cable deal, but now that we have Wi-Fi, I'm pretty much ready to fight anyone I choose.
There are all kinds of reasons, those are just some of the ones I could think of off the top of my head.