Linked from
The 8 pages that link to Endowment effect, each with the reason it gives.
Cognitive biasRelated: Ownership can alter valuation without changing the object itself.
Loss aversionRelated: Ownership can make giving an item up feel like a loss.
Disposition effectRelated: Overvaluing an owned asset can reinforce reluctance to sell it at a loss.
Diderot effectRelated: New possessions can reset what feels worth keeping, replacing, or adding.