Why do people risk their lives for others

an important conversation.

in the episode #70, at the funeral of The Third Hokage

Naruto: Iruka sensei. Why do people risk their lives for others?

Iruka sensei: When one person dies... he disappers. Along with his past, current lifestyle and his future. Many people die in missions and wars. They die easily and in surprisingly simple ways. Hayate was one of them. Those who die have goals and dreams. But everyone has something as important as those. Parents, siblings, friends, lovers... people who are important to you. They trust and help each other. The bond between the people important to you ever since birth... and the string that binds them becomes thicker and stronger as time goes by. It's beyond reason. Those bound to you by that string will do that. Because its important.

Naruto: Yes... I think I understand, somewhat. But... it's painful when someone dies.

Kakashi sensei: The Third didn't die for nothing. He left something important in all of us. You'll find out what eventually.

Naruto: Yeah, I think I understand that too, somewhat.

Iruka Sensei: (thinking) It seems like the small leaves in the Hidden Village of Konoha have inherited that fire of will you spoke of, Third. The fire on the leaves will eventually burn greater and stronger. It will flash its light and protect this village.