Here's an interesting connection between our biology and behavior: Apparently our willingness to risk our life for another person depends on the level of the hormone progesterone in our saliva.
A team of researchers, led by Stephanie Brown and Barbara Fredrickson, have a new paper reporting on the link between progesterone and certain behaviors, like bonding and altruism. Previous research has shown that higher levels of progesterone increase our desire to bond with other people. But according to the new research, it appears there's a feedback loop: Emotionally bonding with others increases our levels of progesterone.
The increased level of this hormone, the researchers say, improves our well-being, reduces our stress and anxiety, and makes us more likely to help other people (even at a cost to ourselves).
As a write-up of the study reports:

According to Brown, the findings are consistent with a new evolutionary theory of altruism which argues that the hormonal basis of social bonds enables people to suppress self-interest when necessary in order to promote the well-being of another person, as when taking care of children or helping ailing family members or friends.