We sometimes say that it "hurts" to part with our stuff even if it's junk, and we know it's junk. Behavioral scientists call it the endowment effect, a theory that people put higher values on things once they own them. Turns out though that it actually does hurt to sell something you own, and it has nothing to do with overvaluing. Stanford University and University of Pennsylvania psychologists recreated the endowment effect in volunteers while scanning their brains with MRI. From Nature News:
If the reason for the endowment effect came from the products being overvalued by their owners, (professor Brian) Knutson's team expected to see a part of the brain called the nucleus accumbus change during the test. It didn't, "whether buying or selling, the activation in the nucleus accumbus looked the same", says (co-author professor Scott) Rick.
But others part of the brain, the insula, which has a role in the experience of pain, and the greater mesial prefrontal cortex became activated when the subjects contemplated selling one of their items. If they had ranked that item as one they particularly liked, the change in the insula was greater.
According to this research, this is because of loss aversion, says Rick. "It is not because people are overplaying the positive [aspects of a possession]." Rather, we just become attached to objects we own – so much so that it takes a lot to convince us to part with them.