From Vaeschanan, by Rabbi Wein

Judaism considers the comforting of others to be an obligatory commandment - a mitzva. The Talmud points out that God Himself, so to speak, came to comfort Yitzchak after the death of his father, Avraham. Thus our tradition of imitating our Creator, so to speak, naturally encompasses this process of comforting others. There are two components of "Jewish comfort." One is the empathy and sympathy expressed to the bereaved by fellow human beings. Sharing the burden is an essential part of comforting others, for it allows a sense of belonging and support to form in the psyche of the one stricken. In fact that is the greatness of faith in the Creator in all such instances - the realization that one is never alone and abandoned. Even "in the shadow of the valley of death... - ...You are with me." Whether through silence or conversation, the sense of the caring concern of others, sustains and comforts. Thus having the ultimate and eternal Other care and understand, at a level far beyond our abilities, our troubles and travails is in Jewish tradition the strong and basic foundation of comfort and consolation. And this idea is reflected repeatedly in the words of Isaiah, where the Lord is portrayed as the ultimate comforter and champion of Israel and even though He has visited troubles upon the Jewish people, His hand of comfort, so to speak, is never far removed from us. All seven haftarot of the weeks between Tisha B'Av to Rosh HaShana are taken from the book of Isaiah's prophecies and the representation of God as being the comfort of Israel is Isaiah's greatest gift to Israel's posterity.