4 ideas
Competition between matched opponents forces each to exceed what either would reach alone, because the existence of a rival sets a moving standard that solitary ambition cannot supply. The feud is not a distraction from the work but its engine, converting personal animosity into productive output.
A sustained rivalry binds two people more tightly than friendship, because each studies the other obsessively, anticipates their moves, and defines their own identity in opposition. The enemy becomes the person you know most thoroughly and cannot stop thinking about.
Reading history through its rivalries and feuds reveals the underlying values a society fights over, since people contest only what they believe matters. The shape of a culture's conflicts exposes its hierarchy of stakes more clearly than its stated ideals.
Once a rivalry takes hold, it generates its own momentum and survives long after the initiating grievance is forgotten or resolved, because the conflict itself becomes the participants' purpose and identity. Ending the feud would mean losing the role each combatant has built around it.