Brand loyalty is a core asset for businesses, yet consumers often switch brands even when no critical issues arise with their habitual brand. Traditional theories fail to fully explain this phenomenon. This study introduces "mainstream fatigue," a psychological effect wherein prolonged exposure to a brand leads to diminished interest and a desire for novelty. Through three empirical studies (N = 1031), we demonstrate that cognitive satiation directly drives brand switching by reducing interest in a brand's functional attributes, while emotional satiation influences brand switching through its interaction with cognitive satiation. Mainstream fatigue mediates the effect of cognitive satiation on brand switching, highlighting the depletion of psychological resources. Furthermore, brand inertia mitigates switching behaviors by counteracting mainstream fatigue. This research provides a dynamic perspective on brand loyalty, extends theories of brand switching, and offers actionable strategies for improving brand management and customer retention in competitive markets...