がる is used to describe a third person's feelings or desires as inferred from their behavior. It is rarely used for the speaker's own feelings because we know our own feelings directly. The form がっている indicates a current, visible state. The present form がる is often used for habitual tendencies or general statements. In conversation, がっている is more common for specific observations.

This pattern is used mainly with adjectives expressing emotions or desires: 怖い, 嬉しい, 悲しい, 寂しい, 暑い, 寒い, 欲しい, and な-adjectives like 嫌, 心配. It is not used with objective adjectives like 大きい or 高い. When using がる with な-adjectives, attach to the stem (e.g., 嫌がる, not 嫌ながる).