Why are we willing to sacrifice ourselves for others

Spoiler alert for "Independence Day" 1996

My wife and I were watching Independence Day, we get to the scene where Russell sacrifices himself to beat the aliens. My wife turns to me and asks me "Why are men always willing to sacrifice themselves for others"?

I told her I'm not sure, I never really thought about it. I mean, I'm sure we all have had a daydream or two where we die a hero or save the day. But I honestly don't know why it's a common thought amongst men. I know for certain I would put myself in harms way to help others, even if it meant death.

Do you ever have those kind of daydreams?

Why do we think that way?

P.S: I'm not saying only men would make that choice to save others no matter what. I know there are plenty of women who would do the same. But I just know way more guys that think like this.