[Explanation]: A metaphor meaning to love a person and, by extension, care about people or things associated with him.
Legend has it that King Zhou, the last ruler of the Shang dynasty, was a corrupt and tyrannical monarch who indulged in extreme luxury and cruelty (see "Helping a Tyrant Do Evil"). Ji Chang, known as "Xibo" (the Chief of the Western Tribes), who later became King Wen of Zhou, was imprisoned for opposing King Zhou. He managed to secure his release only after employing many stratagems. At that time, the Zhou capital was at Qishan (present-day Qishan County, Shaanxi Province). After returning to Qishan, King Wen of Zhou resolved to overthrow the Shang dynasty. He first recruited the strategist Jiang Shang (i.e., Jiang Taigong) as his military advisor, actively trained troops and prepared for war, and annexed several neighboring small feudal states, gradually increasing his power. Next, he moved the capital eastward to Fengyi (near present-day Hu County, Shaanxi Province), preparing for an eastern campaign. However, shortly after the capital was moved, King Wen of Zhou passed away.
Ji Fa, son of King Wen of Zhou, succeeded to the throne as King Wu of Zhou. Jiang Taigong continued to serve as military advisor. King Wu's full brothers, Ji Dan (i.e., the Duke of Zhou) and half-brother Ji Shi (i.e., the Duke of Shao), were two capable assistants. At the same time, King Wu also gained the support of several other feudal lords. Thus, King Wu formally declared a military campaign against King Zhou. The army crossed the Yellow River at Mengjin (a ferry crossing south of present-day Meng County, Henan Province), advanced northeast, and marched directly toward the Shang capital of Zhaoge (northeast of present-day Qi County, Henan Province). Since King Zhou of Shang had lost the people's support, most of his soldiers were unwilling to die for him; many fled, others surrendered, and some even rebelled. Zhaoge was quickly conquered. King Zhou committed suicide, and the Shang dynasty thus came to an end. For the next eight hundred years, the realm belonged to the Zhou, known as the Zhou dynasty.
Shortly after King Wu of Zhou captured Zhaoge, he was uncertain about how to deal with the remaining Shang officials, nobles, and military personnel, and whether the situation could be stabilized. He therefore felt some anxiety. For this reason, he consulted Jiang Taigong and others. The "Shuo Yuan · Gui Fa" (Garden of Persuasions · Valuing the Law), compiled by Liu Xiang of the Han dynasty, records the following passage:
"King Wu conquered Yin, summoned Taigong, and asked, 'What shall we do with his people and troops?' Taigong replied, 'I have heard that one who loves a person also loves the crows on his roof; one who hates a person also detests his underlings. Utterly destroy your enemies, leaving not a single one remaining—how would that be?'"
The general meaning is: After King Wu of Zhou defeated the Yin (Shang) dynasty, he summoned Jiang Taigong and asked, "How should we treat their personnel?" Taigong replied, "I have heard that if you love a person, you also love the crows on his roof; if you hate a person, you also despise his servants and retainers. Let us completely exterminate all opposing forces, leaving not a single one behind—what do you think?"