Endowment Effect

The tendency to value something more highly simply because you already own it.

The endowment effect describes how people demand more to give up an item they own than they'd be willing to pay to acquire the same item if they didn't already have it. In investing, it can make people irrationally reluctant to sell an underperforming asset or stock simply because it's already theirs.

The endowment effect kicked in the moment he inherited the shares, making him value them well above what he'd have paid to buy them himself.