Emotion. Emotional numbing can occur often as a response to extreme trauma. An example would be a combat veteran returning home and feeling nothing at the death of a loved one. People with schizoid personality disorder are indifferent to interpersonal relationships and appear to be emotionless, often. These are just two examples, there are others.
Yes. Post-traumatic stress disorder, depression, schizophrenia, schizoid personality disorder, and antiscocial personality disorder (sociopathy) are all symptoms that can cause a symptom of "numbing" or lack of emotions. A "lack of emotions" or being "zombified" can ALS sometimes even be a medication (treatment) side effect.