Why did Arjuna collapse in dejection and what was Krishna's diagnosis of his weakness of heart?

Arjuna collapsed in dejection because he was overwhelmed by material affection and false compassion, seeing only the bodily relationships of his kinsmen and forgetting the eternal nature of the soul. He lamented the prospect of fighting against his teachers, fathers, and sons, believing that victory would bring only sin and societal condemnation, thus falling into the illusion that the soul is perishable. As Krishna gently chided, "While speaking learned words, you are mourning for what is not worthy of grief" (BG 2.11), revealing that Arjuna's distress stemmed from a fundamental misunderstanding of life and death—a grief that the wise do not share, for the soul is never born nor dies. Krishna's diagnosis of this weakness was that it was a degrading impotence unworthy of a human being who knows the value of life, leading not to higher planets but to infamy (BG 2.2). He explicitly called it "petty weakness of heart" and urged Arjuna to give up this degrading impotence and arise as the chastiser of the enemy (BG 2.3), diagnosing it as a temporary cloud of bodily consciousness that obscured his duty as a kshatriya. The cure was to transcend this false mercy by understanding that the soul cannot be slain, and thus to perform his prescribed duty without attachment to the fruits, thereby restoring his heart to its natural position of strength and devotion.