Why did numbering affect destroyers differently from U-boats?

If U-boats had always used numbers, why would numbering destroyers damage morale?

U-boats had an established numbering tradition. Destroyers had already developed identities around personal names, so removing those names took something away from their crews.

U-boat numbers were part of an established service tradition. Destroyer crews had already known their ships by personal names and formed an attachment to those names. Removing a name therefore changed the identity of a ship that also served as the crew's home. The morale argument depends on that loss of an existing connection, rather than on numbers invariably producing poor morale.