The believer’s heart also needs to have certain amount of sadness and fear. However, even at the times of sadness and concern, his face should be lightened with smile.
Hazrat Ali (KaramAllahu Wajhu wa Alaihi Salaam) says that:
“A believer’s smile is on his face and his sadness is in his heart.”
In other words, a mature believer becomes sad and pours his tears down in deserted places when he remembers his faults. Their tears are also the source of the light in their faces. Jalal al-Dīn Rūmī قدس سره explains the wisdom of the light in a believer’s face as follows:
“O idolater (dualist), (if) I do not behold my own face (reality), I behold your face and you behold mine. He that beholds his own face (reality)—his light is greater than the light of the creatures (of God).” Mathnawī, II, 883-884
In other words, one of the reasons of the light of a believer’s face is to be able to look carefully at one’s own face and see his own faults before seeing other people’s mistakes. This is the source of the light in the face of a gnostic, who comprehend the wisdom behind the saying “Those who know themselves know their Lord.”
-An excerpt from “From the exemplary manners of the Friends of Allah.”