Hoarders have their own logic. Their logic is very clever and well thought out. As a professional organizer specializing in chronic disorganization I found that the best way to work with hoarding clients is via coaching and it also helps if they are in therapy as well. This is because the stuff they compulsively collect and keep is really just a symptom of what's going on inside them.
Building a Relationship with the Hoarder
Before they will listen to what someone else has to say, they have a VERY deep need to feel understood and appreciated. They live in constant fear. Fear of bad things happening and not being prepared, fear of being "found out" and misunderstood.
They have very little trust that their needs will be taken care of. They feel utterly vulnerable and unsafe in the world, like no one understands them.
Before they will drop their guard and listen to someone else, they need to be honestly appreciated and acknowledged for the positive intention behind their every behavior, no matter how odd it seems to you. They need to feel understood.
The Hoarder's Logic
Their logic is all about preventing and/or being prepared for the worst case scenarios they constantly imagine. The flaw in their logic is often that they haven't factored in the real likelihood of the worst case actually happening. They also may not have considered other possible ways to be prepared and create the same feeling of safety - without so much stuff.
Often, they have not fully weighed the benefit of their solution against the true price they are paying to maintain that feeling of safety. They also usually do not give themselves credit for the amount of resourcefulness and creativity they have in dealing with situations as they arise. Learning that they actually can survive painful emotions or "worst case" scenarios is a really big part of working through hoarding as it is with most addictions.
The Role of Beliefs and Trust
Working with hoarders requires a great deal of patience and commitment to really fully and deeply understand how their systems benefit them, and what the root beliefs and emotions are that underly the logic. Challenging those core beliefs is a risky endeavor though. It is important to be aware of the signs that you may be touching on such deep beliefs that therapy may be indicated to ensure they rebuild a strong sense of self.
An effective relationship with a hoarder requires that they truly trust that your only only intention is help them have what they want and that you don't have a personal agenda - like throwing all their stuff away just because YOU don't like it or want it.
That's why it is so difficult for family members to help a hoarder overcome the problem.
How can you help someone when their behavior is so personally painful to you? You can't help but have an agenda for your own gain.
The Role of Trauma
What you see is often the physical evidence of very deep past trauma. It may be OCD, but there is also a lot of evidence that most hoarding is not actually OCD. (See "Buried in Treasures" by Tolin, Frost and Steketee and "Treatments that Work: Compulsive Hoarding and Acquiring" by Steketee and Frost.)
It is often a very deeply "addictive" process in which things are used to "soothe" the pain of past traumas and to cope with "anticipated pain or danger." Some therapists call this "traumatized attachment". It can be caused by abuse, loss of something you cared about, death in the family, neglect, being frightened by adults, by being constantly subjected to not having "enough" (as in the poverty that came with the depression") and many other situations.
Interestingly, the "poverty mentality" can occur even in people who have never actually experienced poverty. The fear of poverty can be just as debilitating as actual poverty.
In all these cases, the things they collect make them feel "safer." That said, some people are genetically predisposed to having a stronger need to "gather" things, just as some people are more likely to experience depression than others as a reaction a trauma. Trauma affects everyone diferently because there is a genetic element to it, but genetics is not necessarily the sole "cause."
I could go on, but I'll stop here. There are no short answers to the problem of hoarding. It is so varied. But rest assured, if someone you love wants to change, or if they want to understand what is causing them to feel such a deep need to hold on to things, there is hope. The thing is that people have to be ready to change and ready to accept help.
In the meantime, taking care of yourself and continuing to learn and grow your understanding is the best thing you can do.
I wish you all the best,
p.s. I have a whole page on my blog with resources to help hoarders at http://blog.neatandsimple.com/help-for-hoarders-clutter.html