Just where does that childish greed for presents come from?

Christmas is going to be a lot of fun this year, I can tell.
When you ask Clay what he wants for Christmas, he'll say "a tree."
He's been excited about getting a Christmas tree since he saw one decorated with toy trains, cars, etc. in a store back  in...August, maybe?
When his momma explained that there might be toys under the tree at Christmas-time, his eyes got even bigger.
So today, when we were shopping in a big box store, Clay saw some things he wanted as we walked through aisles displaying toy train sets, giant stuffed animals and other items that looked pretty tantalizing to a three year old.
"Clay have that, take it to Nan and Papa's house," Clay politely suggested a couple of times.
I told him he'd have to add it to his wish list, and maybe Santa would get it for him.
And just like that, here I go perpetuating the whole materialistic Christmas paradigm.
A part of me resists the idea, yet another part of me gets as excited about it as...well, a kid at Christmas.
And to think that Clay probably would have been satisfied with just a tree.