That is a good question. My mom was not a hoarder, but he was. I am and I am in constant battle with myself to toss out items. Some pose sentimental value but others just mark a time in my life. I think it is a mental state some of us get into and i think depression has something to do with it. I don't believe that behavior has anything to do with the how or why.
I am always battling depression of a sort. It can really get you down, and I feel a sense of security by things I have around me.