The Tzemach Tzedek, after his rebbitzin passed on, refused to be comforted, and engrossed himself in mourning. One of his relatives approached him and quoted the teaching of the sages that even with one's wife, when she passes on, it comes from G-d, and it is Torah. As soon as the Tzemach Tzedek heard this, he said you have revived me, and began to feel good.
Why was it necessary for this relative to tell the Tzemach Tzedek this, did he not know the teaching of the sages? He surely did. However, there is a difference between knowing something in thought and hearing it in words, although the idea is identical. A person who just had a loss is considered a captive to sadness; it is impossible for him to remove himself from his captivity, because one who is captured cannot get out of his own jail. He needs someone else to help him leave jail. This applies to the greatest person, including a Rebbe; he needs a relative to come, and verbalize comfort alleviating him from his sadness.