I can count on my fingers and toes the number of people that I know who have a real hard time throwing stuff away. My Dad for one, he gets mildly obsessed with things. For example, he recently got into golfing, he now has three full sets of golf clubs plus dozens of random clubs that I can't trouble myself to know the names of. Dad, you only need one set of golf clubs - lets get serious.
Stuff: Compulsive Hoarding and the Meaning of Things by Randy O. Frost and Gail Skeketee, is a great book for helping people understand why they hoard, or in a lot of your cases, why a family member or friend hoards. The authors personally treat people who suffer from compulsive hoarding. Once you discover why you hoard, (there can be many factors), you can work on fixing the problem.
In the book they do explain that there is a ways to tell if you are a hoarder. There is a difference between hoarding and collecting. Lets go back to my Dad for a minute - It's considered hoarding when objects do not have any use anymore. My Dads golf clubs are extremely useful, but he still doesn't need 30. Hoarding is usually cluttered and not organized. My Dad keeps his golf clubs in his shed, where they are completely organized, which is a sign of a collector. Okay, Dad you win this round, I guess you are not as weird as I thought.
You may hoard and you don't even realize it. Frost and Steketee are completely non-judgemental and 100% here to help. To avoid an intervention of my own, I am going to work on cleaning out my closet. Stuff is real eye-opener, no matter how hardcore of a hoarder you are.