理该 is an adverbial phrase meaning 'rightfully should' or 'naturally ought to.' It expresses a strong sense of moral, logical, or social expectation — something that is not merely desirable but objectively justified by reason, duty, or common sense. The character 理 (lǐ) means 'reason,' 'principle,' or 'logic,' while 该 (gāi) means 'should' or 'ought to' — together, they emphasize obligation grounded in rationality or fairness, not just personal preference.

This phrase is formal and somewhat literary; it appears more often in written Chinese (e.g., essays, official statements, or solemn speech) than in casual conversation. It carries a tone of quiet authority or gentle reproach — for instance, implying someone has failed to meet an obvious standard. It’s commonly used in contexts involving responsibility, justice, etiquette, or natural consequences, such as acknowledging due recognition, fair treatment, or expected behavior in daily routines or social roles.