There is a profound, constructive power in expressions of anger, strictness, or criticism that far outweighs the benefits of superficial joy or complacency. Rather than causing harm, a stern or troubled face can actually transform a person's heart for the better, refining their character and correcting their life's direction.
The primary approach among commentators views this dynamic through the lens of God's relationship with humanity. It is ultimately better for a person to experience God's strict justice and anger in this world than to enjoy a peaceful, carefree life while continuing to sin [רש״י, תורה תמימה, תעלומות חכמה, צאינה וראינה]. When God brings punishment, a person should not fall into despair. Instead, these hardships serve to awaken the heart to repentance. If God had shown an angry, strict face to the generation of the Flood and the people of Sodom, rather than granting them economic abundance and peaceful lives, they might have corrected their ways and avoided destruction [רש״י, תורה תמימה, צאינה וראינה].
This principle also carries immense practical weight in raising children. King David, motivated by love, avoided showing anger or causing sadness to his sons Adonijah and Amnon, a choice that ultimately led to their tragic downfalls [רש״י, תורה תמימה]. In parenting, it is often much healthier for a father to show a stern face and establish clear boundaries than to offer constant affection and laughter. Presenting a strict expression can effectively correct a young person's behavior, completely removing the need for physical punishment [חומת אנך].
On an interpersonal level, this concept highlights the deep importance of honesty. When a person hurts a friend, it is far better for the victim to show genuine anger than to hide behind a fake, cheerful expression. Expressing anger releases pent-up frustration, prevents future revenge, and cleanses the heart, whereas a forced smile only masks a suppressed hatred that will inevitably explode later [מצודת דוד]. Furthermore, receiving honest criticism and facing another's anger provides a valuable tool for a person to improve and develop their own character [ביאור שטיינזלץ].
From a more philosophical perspective, this dynamic mirrors the internal struggles of the human mind. The heart represents intellect and wisdom, which are tasked with overcoming physical urges and desires. The difficult process of restraining these lower desires naturally brings about a sense of worry and seriousness, reflected in a stern face. Yet, it is precisely this challenging process that purifies the mind and guides a person toward genuine internal perfection [אבן עזרא].