My wife and I had seperate life changing experiences with hoarders a couple of years ago.
Back when we were both still attending university my wife had a "job" helping a nice-but-crazy woman sort out her things. What started as taking care of her rental properties (she had inherited quite a bit from her mother and bills) quickly turned into trying to save a hoarder from plunging into complete disaster. Said woman was crazy in a esoteric kind of way, never having really worked for her living (she described herself as a self-taught dance therapist) and massively in debt. Her mother's property was what kept her head floating right above the surface but she spent tons of money on useless junk. When one of her houses had to be renovated she was there to help throw stuff out but instead ended up shuffling junk around from one room to another. And we're not only talking about small items such as used and empty pens that she kept in her own apartment, we're talking about kitchen appliances from the 50s that were probably a health and safety hazard for the entire neighborhood. I think ritchie70 hit the nail on the head: inability to determine value.
Around the same time I was helping my dad move an older lady, sister to my mom's best friend who'd died of cancer a couple of years ago. This woman had been living in the same apartment for 30something years, having moved to Germany from Canada. There were still unopened boxes in her basement with stuff that she had decided was worth moving across the Atlantic ocean more than 30 years ago. Her entire apartment was filled with junk, so was her basement and the additional storage unit she'd rented. And she still valued all these things highly even tough she hadn't seen some of them for more than 30 years! We weren't allowed to throw anything away, like old dresses, used shoes with holes in them, knitting magazines from the 70s, an old TV that had stopped working a long time ago, etc.
In the evenings, when my wife and I were having dinner, we'd talk about the events of the day and quickly realized how amazed and disturbed we both had been by the experience. This has led to some massive clearing out on our part. From that point on we'd decided only to keep around so much stuff that we could easily move anywhere on this planet the very next day. Our friends have already started by wondering how empty our apartment had become over the last few years but to be honest we're not missing anything. We've also completely stopped buying new random crap simply because we like it, or because it's nice to have or to look at (that was long before I found this website). Also, every two to three months we go through our aparment and look for things we hadn't used in a while. I'm afraid that my parents on the other hand have gone the exact opposite way. All the stuff said woman did allow us to throw away my dad took, and it's still in his basement. He now has some old 60s cameras, radios and whatnots that don't work and are worth less than a zero each.