This could apply to most games I've played also.
Today I just realized: I've played more Pokemon White than any friggen game in my collection. 300+ hours and I'm still not getting bored of it.
It also made me think: why does no other game have replay value of this magnitude? I played and beat Mario Kart 7 and tried to get into it, but I realized how horribly flawed this whole game is: you are bombarded with car-stopping items, and Wi-fi is the biggest load of crap: players literally WARP to the end of the track. The more I play it, the more I hate it. Pokemon, on the other hand, has the potential of raising over 640 Pokemon, and I feel the potential to train them all to their best potential. And then I feel like training multiple ones of each for different teams, and the Pokemon themselves become inspirations for me and my artwork, and help me with character development/form/size, etc. This game has also helped me make some very good friends. Thanks to this game, I really can't enjoy any other game anymore. My standards on games have also changed too: often ask myself if I'm really going to be playing this game for years on end. I can only apply to that to, like 10 of the 200+ games I own.
I don't know how I'm coming to this today, I just feel like I'm not enjoying games that much: the bonus content just doesn't seem engaging to me anymore.

If you like RPG's try to get Final Fantasy Tactics A2: it's a great game which is very lengthy too.