From what I can tell, as merely an observer, people tend to hold the first pokemon game they ever picked up closest to their heart. If your first game was Diamond/Pearl, you might find yourself more drawn to the game that introduced you to the Pokemon world.
Which is why I don't understand why Gen I enthusiasts get so much hate. If Red/Blue was the first Pokemon game they ever picked up, why should they get flamed for loving Gen I more than the later generations? It's only natural. But people seem to think they love them because they're pricks who think the originals are always better. But speaking as a first gen lover, I can honestly say I only like those Pokemon better because they remind me of my childhood, and the anime, and all of Ash's adventures when I was a kid. They're the Pokemon that I know down to every last detail, and yes, even now I still find myself singing the Pokerap every now and then.
I just think the amount of hate people get for preferring Gen I Pokemon is ridiculous. It's not a matter of being snobby, it's a matter of nostalgia...