The inner world of a human being is a deeply personal experience, entirely hidden from the eyes of those around them. The primary approach among commentators is that human experience is fundamentally solitary. A person alone directly feels their own pain and sorrow. Just as no one else can truly grasp the depth of another person's bitterness, no stranger can share in their inner joy with the same intensity [ביאור שטיינזלץ, עמנואל הרומי]. Because a person's true intentions remain buried deep inside, it is impossible to know exactly what is happening in someone else's heart. This hidden reality naturally calls for a degree of caution when building relationships with others [אמרי דעת].
This sense of isolation also plays a significant role in human psychology. A person overwhelmed by worry and internal bitterness often withdraws from society, becoming fearful of others and unable to join in their happiness [אמרי דעת]. Similarly, feelings of guilt and joy are profoundly private. The heart of a sinner constantly feels the restless bitterness of guilt, while upright individuals experience a hidden, internal joy that no outsider can possibly understand [מלבי״ם].
Beyond personal psychology, this concept applies to spiritual effort and its ultimate reward. Bitterness represents the hard work and spiritual toil invested in studying God's teachings. Because a person struggles and works alone in this world, no stranger will divide the reward and joy they will receive in the future [רש״י, מצודת דוד]. On a national level, the people of Israel intimately know the bitterness of exile and the sacrifices made for the holiness of God. Consequently, the other nations of the world will not partake in the future joy of Israel [רש״י].
From a moral perspective, a wise heart recognizes that physical, worldly pleasures are actually bitter and harmful to the soul. Therefore, such an individual avoids physical indulgences and chooses not to participate in the material celebrations of society [אבן עזרא, חומת אנך, אמרי דעת]. Philosophically, the human mind recognizes the struggles of the lower, animalistic instincts, yet those lower faculties cannot share in the pure joy the mind feels when it achieves spiritual understanding [רלב״ג]. Finally, there is a warning against excessive joy, which can easily lead to sin. If a person remembers the bitterness of their past wrongs, their happiness remains grounded and restrained. Through this humility, the ultimate stranger, which is the evil inclination, is prevented from infiltrating their joy and causing them to stumble [אלשיך, חומת אנך].