What makes a martyr?

What kind of person sacrifices everything for a stranger? What would make a man or woman give up everything--family, friends, home, comfort, even their own life--for a person or a group that he or she has never met and might never meet?

What does it take to become a martyr? A real martyr--not a poseur, someone who shouts "Look at me, am I not wonderful, am I not the morally superior! "--but someone who honestly pursues a goal without caring about the cost to themselves or to the lack or any reward; what makes such a person?

I have an image of a character in my mind--a man who gives up everything, travels far from home, and great risk to his own life and sanity, all for a group of people who he has never met, who may regard him with hostility (if not outright violence)--and I am struggling to understand the why of this character. If I don't know the why, how can I know what he will do during moments of crisis?