I think theres also a generational aspect, and an economic aspect. Older people tend to be less comfortable throwing things away (usually not a bad thing), but so few things are designed to be saved or even recycled, these days. Add people who grew up poor... things become more valuable, because they represent resliance.

I think there needs to be a distinction between the kind of hoarding i just described, and the mental illness that leads to people saving everything and anything. Like @bleurowz described, that kind of hoarding is much harder to understand, for people who havent really seen it or lived it, and its just as useless to expect those hoarders to "snap out of it", than it is to tell a depressed person to "adjust their attitude".

I have a friend whose sister just died of brain cancer. I've helping my friend clean out her sister's apartment.

The sister was a hoarder, an end-times christian prepper, and a right-wing radio talk show listener. I personally carried out more than 40 cans of food over 10 years out-of-date. The sister saved used plastic gloves and used plastic forks. I filled a laundry basket with ballpoint pens. She also had camping gear, portable stoves, raincoats, ... all the stuff you need to take care of your people who apparently missed the rapture.

The place stunk so bad I had to wash my clothes when I got home. It has motivated me to clean up my own place, lest some of my friends be subjected to a similar experience.

Hoarding is really a physical manifestation of something serious going on emotionally. It's really not enough to clean things up, but to deal with what triggers the behavior in the first place. There's a lot of overlap with addiction and OCD... I'm not a hoarder per se, but I'm certainly a clutterbug and I tend to hold on to too many things long after they have no meaning or use for me. I've managed to make inroads with this, but it hasn't been easy. There is a skewered feeling of control that comes with acquiring and holding on to things, especially for someone who may fear their life isn't in control -- traumatic experiences and/or overwhelming emotions that feel too daunting to face or learn to contain. I've had to own some very uncomfortable things about my life; doing so ironically gave me the freedom to let go of a lot, both physically and emotionally.