In Shakespeare's "King Lear," the poignant line "How sharper than a serpent's tooth it is To have a thankless child!" captures the intense emotional pain that a parent feels when their child shows ingratitude or betrayal. The metaphor of a serpent's tooth emphasizes the deep cut that such disappointment can inflict on a parent's heart. This sentiment reflects the themes of familial relationships and the struggles that arise from expectations versus reality.

The play explores the complex dynamics between parents and their children, particularly focusing on Lear's relationships with his daughters. As Lear descends into madness, his realization of ungratefulness sheds light on the universal agony parents face when their sacrifices go unappreciated. This line encapsulates the heartache that comes from love that is not reciprocated, illustrating the fragility of familial bonds and the tragic consequences of misunderstanding and misjudgment.