What makes heros?

What makes heros our of individuals in times of disasters, dangers etc.

What made the Taj chef help people, losing his life? What made the nanny run into the house to save the 2 year old kid? They went beyond their job scope to help save people, putting their own life at risk.Nothing would have prepared these people for situations like this? They arent very educated or anything.

This question has been bugging me for quite some time. What makes heros out of simple individuals?