Why do people risk their lives for others?

From Uzumaki Naruto (<span lang="ja">うずまきナルト</span>):

Uzumaki Naruto: "Iruka-sensei (うみのイルカ), why do people risk their lives
for others?"

Iruka-sensei: "When someone dies, they disappear. Their past and present,
along with their future. A lot of people die on missions and in wars,
surprisingly quickly and easily. Hayate (月光ハヤテ) was one of them. The
one who dies has his own dreams, but there are people who are treasured like
those dreams. Parents, siblings, friends, and lovers, people who are
important... They all believe in and help each other. People who were there
since birth took care of them... And those bonds grow even stronger as time
passes. It's not just a saying. It will be so for the ones who are
connected, because it is precious."