Economist Richard Thaler was the first to came up with the endowment effect theory. He theorized that people tend to value goods that they own more than those they don't even if the goods are both identical. In an experiment, those who are given a mug consistently price it higher than those who are asked to just value the mug directly without owning them. This is a cognitive bias and seems to be related to status quo bias where we prefer things to remain unchanged.

This is probably the reason why I'm unable to sell off most of my unwanted things and leave them rotting somewhere in the house.