Portraits are always a challenge particularly if you are interested in realism. There is little point in an exact copy of a photo but there is definitely something a little bit special about a drawing that has photo-like qualities. I feel a connection is forged between artist and subject thanks to the many hours of intense observation and interpretation.
Perhaps the use of light and shade can shift emphasis in a subtle manner to bring out a certain aspect of personality......... I don't know ......but it is very satisfying to try.