Why does classic feel so rewarding? Its like an addiction

I do the same quests I have done so many times. I see the same areas I have seen so many times.

And yet I feel like I am addicted to classic. I want to play. I want to do the next quest, hit the next level and so on. Why is that so?

With Cataclysm, I dont have the same feeling. Sure, I love my hunter and want to raid Desthwing. I am happy, if MoP hits. But I can stop playing cata for so long.