When adding new labels to the "Filter by Label" option in the Library we only allow lowercase letters to be used. This function helps keep users from creating two labels that are the exact same. 

For example, let's say you create a label filter called "disney." By making every label in lowercase letters, this will help prevent you from accidentally creating "Disney" and "disney" as separate labels.  Which allows you to search faster and you don't have to worry about how you spelled the Label originally!