• As time passed, he just became used to the mask as any other thing in his life. His mask became a part of his identity hence he didn’t stop wearing it.
• Last but not the least, he probably wears the mask to stop nose bleeds which he gets while reading eechi eechi paradise.

These are the possible answers to the question why Kakashi wears a mask. We may or may not know the true story in the future, until then let’s theorize and have fun.

~Dattebayo

RELATED CONTENT

Does Kakashi have Sharingan after the war?

Does Kakashi get married and have a kid?

What does Kakashi read?