OCD, mostly. I'm compelled to catch 'em all.
But I also like looking through the games and finding all the little flavor-texts and things that they put in. I'm not really interested in the game's stories, because I can use the attitudes of my Pokemon and the information from the Pokedex to create my own. It's the easiest medium for me to write off of, as there can be so much drama and action in a story that involves sentient creatures and the trainers who work with them to accomplish goals.
That, and each new series has a new set of Pokemon that I like. I started off falling in love with the game ever since I saw a picture of Charmander in a Nintendo Power, and haven't looked back since. If I get bored with a game, I simply find a new way to play it. Tired of battling? Train for some contests! Tired of contests? Put my Pokemon on the Wii Ranch and see how they interact with the other legendaries.
Tired of all of it? Start a new game and write a day-by-day story based on the whole adventure from the eyes of the trainer himself, seeing how I'd 'play' myself if I were in the game. I find it makes getting more than six Pokemon difficult, because after I look at the natures and traits of the Pokemon and put them into my story in some defining way (such as having my Piplup jump into the nearest fight he can get his flippers on, or having my Machop, who likes to clown around, juggle the stones he breaks off of a boulder after Rock Smash to the delight of his 'adoring' fans), I get attached to them.
It must be how Ash felt, I suppose... but playing it like that has certainly given me a new way of looking at the game.