As he was about to graduate from Yale, John Kerry volunteered to serve in Vietnam - because, as he later said, "it was the right thing to do." He believed that because he had had a lot of privileges in life - for example, attending a great university like Yale - he had a responsibility to give something back to his country. His two great heroes - his father and John F. Kennedy - had served during World War II. John Kerry wanted to follow both their example and that of some of his best friends, who also went to Vietnam.
John Kerry enlisted in the Navy in 1966. After completing Naval Officer Candidates School, he began his first tour of duty on the USS Gridley, a guided-missile frigate in the waters adjacent to Vietnam. In 1968, John Kerry began his second tour of duty, and volunteered to serve on a Swift Boat, one of the most dangerous assignments of the war. Swift Boats patrolled the narrow inlets and canals around the Mekong Delta "to draw fire and smoke out the enemy," according to the The Boston Globe.
To the men who served with him, John Kerry was "one of the most daring skippers in the US Navy, relentlessly and courageously engaging the enemy," according to The Boston Globe. William Zaladonis, who served under Kerry on a Swift Boat, said that he was more interested than some other commanding officers in his crewmates' lives, hopes, and dreams. John Kerry put his crewmates at ease, telling them to "call me John - you don't have to call me sir." When "we were out on patrol," Zaladonis recalls, "we were a family."
http://www.johnkerry.com/about/john_kerry/service.html