How does a lack of emotion occur?

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.
Depression or anger. This may be delayed grief, or perhaps a full blown depression. There may be suppression of emotions as it may be too painful to allow oneself to feel them or to show them. If the relationship with the decease has been conflicted, this may also happen more often than not.