I feel that some people play the game for simple enjoyment, some to collect, and some to see what they can do with their town. If someone has worked very hard on their town or spent a lot of time collecting items, then I believe they would be more protective of that. I don't think everyone is being greedy or rude, etc. The last person I played with gave me a pinwheel, and I gave them the cowboy boots they liked in return. You just have to find the people who play the game in the same way you do.
I guess it all depends on what each person finds fun. For example I'm a completionist. I like getting all the items, the badges, filling up my encyclopedias, and I like planning out my PWP's and my town layout to be as nice as I can make it. And to some people it might seem as if I'm taking the game too seriously, or however people might want to put it.
But to me doing all those things is very enjoyable. I like making lists and ticking off items I've gotten, or things that I've done. So to me I'm not taking it seriously at all, I'm just playing the game how I want to play it, which is what is so neat about AC, it's a game that can be played and enjoyed in many different ways.
I think there are just a lot of people on TBT with certain prefered play styles, and because TBT is so good for getting items and villagers and such, it seems as if those are the only ways people play the game. Where as people who have a more laid back way of playing might not go hunting in the villager trading plaza or at Re-tail that much? That's just a guess, but I think you can understand what I'm getting at.