Back when Celestia was the teacher of a certain bratty amber Unicorn, she also had another student.
He wasn't particularly skilled in magic nor was his magic powerful like a later student that she'll have in the future.
But he had a certain strength to him, no matter the lesson or how difficult it was, he persevered no matter what.

Celestia took interest in this young stallion and when a certain other student went rouge, she decided to give this stallion a special reward for his perseverance.