The answer to that question might be found in Murphy's upbringing on Long Island.
A blue-eyed young man of Irish stock, Murphy's friends and relatives were public servants - cops and lifeguards, firemen, teachers and criminal court officials. His hometown of Patchogue was a working- and middle-class community of third- and fourth-generation Irish, Italian and German immigrants. His father's father was as Irish as a man can be while still being an American - he was born aboard a ship from Ireland as it steamed into New York harbor.
While he grew up, Murphy watched as those closest to him tried to right the world. One night when Murphy's father, Daniel - then an assistant Suffolk County district attorney - thought his 10-year-old son had gone off to bed, he began prepping for a murder case he was to prosecute the next morning.
Two defendants were charged with killing a man and dumping his body into a cesspool behind a Patchogue auto parts store. The senior Murphy laid gruesome crime scene photographs across the kitchen table.
Then Michael wandered in.
"He asked me what were the pictures of?" Daniel Murphy recalled. "And then he threw up."
A few weeks after Michael graduated from Patchogue-Medford High School in 1994, a sniper firing from outside a Commack diner killed a 50-year-old lawyer as he sat with his wife waiting for their dinner. Three days later he shot at a gasoline station attendant. Three weeks later, the sniper shot a 43-year-old waitress on the night shift at a fast food restaurant, wounding her in the lung.
With Long Island in a panic, police had nothing to go on but a few .35- caliber bullet fragments. One of Murphy's relatives, John J. McElhone, deputy chief of Suffolk's detective squad, ran the investigation. Four months and 1,600 interviews later, they caught and eventually convicted the sniper, who was sentenced to 15 years to life in prison. Michael watched the drama unfold.
"Our whole family is police, firemen, lawyers, people helping other people," said Michael's mother, Maureen Murphy, a cousin of McElhone's. "He realized that's just the way you grow up."
His father said even as a young child, Michael "never liked seeing people getting taken advantage of or getting picked on."
While Michael was a pupil at Patchogue's Saxton Middle School, he jumped into a fight to help defend a classmate who had been cornered behind the building. After that incident, Michael couldn't shake the nickname: "the protector."
"I think he liked the fact that there were people out there trying to protect other people and trying to solve the world's problems," Daniel Murphy said.
While there, Murphy began to demonstrate qualities that made people around him willing to follow his lead, say former lifeguards who worked with him.
Friends from his high school and college years describe Murphy as someone who others naturally gravitated to - who exuded loyalty and instilled loyalty in those around him.
When he was 16, his was the larger of the two bedrooms that he and his brother John occupied. But when an uncle fell on hard times and needed the Murphy family to help care for his three daughters, Murphy volunteered to take a smaller spare room, rather than ask his little brother to move.
That day on the Afghanistan mountain, Murphy wore a large red sleeve emblem in honor of a former lifeguard he worked with - Owen O'Callaghan - who later became a fireman at Spanish Harlem's Engine 53 Ladder 43.
O'Callaghan, now a Suffolk police officer, said when Murphy set his sights on joining the SEALs, he set up a chin-up bar at the beach, and prodded his fellow lifeguards to get stronger with him.
"I'd be, like, 'I don't want to do any more,' and he'd say, 'You can do it'," said O'Callaghan, 28, who credits Murphy with preparing him to meet the physical requirements to become a fireman.
"He wouldn't let you quit."