Hoarding is not yet a certified psychological condition though its symptoms are well known and follow consistent patterns. Like most psychological orientations, hoarding exists along a spectrum, from someone who simply likes to save things to a person who is unable to get rid of anything. Most philatelists tend to be in the lower range of the hoarding spectrum, enjoying keeping things but perfectly capable of pruning their holdings as needed.
Though the underlying psychological reasons for hoarding are often not clear, I think the experience of our hobby in the last thirty years illustrates that hoarders can’t part with things because they believe that the objects that they part with may someday be of value to them. We used to see far more hoarding in our hobby. I would constantly be called to homes where there was wall to wall, room to room stamps with nothing ever being thrown out. I think this was a overreaction to the great price rise in stamps that took place from 1930-1980 where ordinary common postage stamps of the 1920s began to sell for real money by 1980. Collectors just didn’t know what was going to get good, and people with hoarding issues to begin with were paralyzed by not knowing what to save and so they saved everything.
In a sense the lack of price movement in stamps for the last thirty years has been therapeutic for philatelic hoarders. Since so little has risen significantly in value the inclination to hoard has received very little real life validation and so only the most pathological have continued to obstruct their homes with stamps.