The "Will of Fire" (火の意志, "Hi no Ishi") is a recurring element throughout the Naruto series. It is an ideal, and the life philosophy Hashirama Senju lived by and held, which has since been passed on to many ninja from Konohagakure as a part of their spiritual heritage. The 'Will of Fire' states that every true Leaf shinobi must love, believe, cherish and fight for the village's sake, as many previous generations had done before them.
According to Hiruzen Sarutobi, the "Will of Fire" is what gives the shinobi the strength to continue fighting against all odds, building willpower and strength of character. It is also symbolic of the hopes and dreams of the previous generation being passed to the next; for example when Asuma Sarutobi was killed, his hopes and dreams were passed to Shikamaru Nara, who would in turn pass it on to Asuma and Kurenai Yuhi's child.
According to Madara Uchiha, the hatred of the Uchiha clan is destined to clash against the Will of Fire, as he believed that Sasuke Uchiha and Naruto Uzumaki are destined to fight. There have been exceptions Itachi and Obito both valued the will of fire over hatred and died protecting their friends and the village.