Final answer:
George is occasionally frustrated with Lennie because Lennie's actions unintentionally hinder George's plans and threaten their shared dream.
Step-by-step explanation:
George's occasional frustration with Lennie in John Steinbeck's novel Of Mice and Men is primarily because Lennie hinders George's ability to do some things. Lennie's intellectual disability and lack of awareness often lead to situations that complicate or threaten the pair's employment and safety. Although Lennie does not have ill intentions, his actions can provoke unintentional consequences that frustrate George and jeopardize their shared dreams. George cares for Lennie, but the responsibility of looking after Lennie weighs heavily on him, causing moments of frustration when Lennie's behavior disrupts their plans.