Why do some people have no sense of humor?

This person I know is so nice, but she has no idea what humor is. She takes everything so literally, and she never understands the slightest kind of humor. Obviously irony is lost on her as well. Is there a definition for people like this? She comes across as socially awkward. She’s a special ed teacher and it affects her job because she can’t relate to parents or students as well as she could if she understood humor. Can anyone give me some insight? Thanks.