Why was Brightheart first named Lostface by Bluestar?

Brightpaw's face was badly scared by a pack of dogs, and Bluestar, believing that Starclan had abandoned her, gave Brightpaw the name of Lostface to show what Starclan was doing to her clan.

When Bluestar later regained her faith and Firestar became leader of Thunderclan, he changed Lostface's name to Brightheart.