When talking about ninjas, both in real history and manga/anime, loyalty is always considered a core value.
The Will of Fire is the philosophy that the First Hokage, Hashirama Senju, followed as he believed that love and loyalty would make peace.
However, Naruto overcame everything and later became the Seventh Hokage, the hero representing the will of fire to change prejudice from Konoha village, and was admired by everyone.
To talk about the origin, the foundation that forms the will of fire, it originally originated from Asura, the son of Sage Hagoromo Otsutsuki.
So this feud continued through the next lives of both Otsutsuki brothers.
It was Madara and Hashirama who joined hands to form Konoha Village.
The will of fire seems to represent the glorious tradition of Konoha village, while there is a strong bond between people.
Naruto showed Sasuke that trust and love can help a ninja become stronger, instead of pushing themselves deeper into hatred and jealousy.