What happened to Gin Ichimaru's that made him have such an evil grin?

Answer:
We don't know what occured, but as he has had the grin since he was a young child we can assume this is part of his original character
First answer by Ashesfalllikesnow. Last edit by Ashesfalllikesnow. Contributor trust: 2 [recommend contributor recommended]. Question popularity: 1 [recommend question].