理由 refers to a reason or justification people give to explain or defend an action or opinion (e.g., 你有什么理由？). Its sibling 原因 refers to the cause or objective reason for an event (e.g., 失败的原因 'cause of failure'), while 道理 is the logical principle or sense behind something (讲道理 'reason with someone'). Choose 理由 when you mean a stated rationale, 原因 for a factual cause.