I think its a matter of what motivates people. The mother lifts the car off of the child because she has the maternal instinct to save the child she birthed. I think some people are highly motivated to help people and it fulfills their soul. They do heroic deeds because it makes sense to them. Its all about motivation.
"What’s the psychology of heroism? Is extreme self-sacrifice the result of a pained calculus, a weighing of desire and obligation, or an instinct? (Which would be more heroic? Does it matter?)"
http://www.slate.com/articles/health_and_science/medical_examiner/2014/10/psychology_of_heroism_and_altruism_what_makes_people_do_good_deeds.single.html