Wow @kaylinart, you just took these words from my mouth. I think it's in human nature to be possessive and it's that habit of possession that makes people keep things they don't need.
I am a victim of your second point though which is duplicating what I have. If I have a red shirt that I like so much, I would love to have the blue, the black and so many other colours of it and just when I am done buying them and probably wearing them Ines, I become tired of them and dump them. But one thing about me is that I always look for someone to give. So it isn't really bad getting those things you feel you like in excess but it becomes bad when you don't give them away to those who need them most after you have gotten tired of them...