There are many diverse views on this question. At the time, it was argued that the US had "a promise to keep" to the South Vietnamese (to "support" them) and to the world. "There are people whose well-being rests on the belief that they can count on the US." President Johnson wanted people to think the US was saving the world from the threat of Communism, trying to keep morale high. This was said a month after the start of Operation Rolling Thunder, and he was obviously trying to convince people that war is justified and "worth it".
Some raise doubts about whether Vietnam was really worth it, but argue that once the war had begun it became impossible to "get out". So although the Communist threat was perhaps not as threatening as the President wanted people to believe, "to leave Vietnam to its fate would shake the value of an American commitment.".
Other suggest that the "rhetoric" about protecting the South Vietnamese was a lie, and that actually "the US didn't want an independent South Vietnam that was no longer dominated by America" because "that would undermine American influence in the area." Critics of the war, of course would not say anything that would support America's reasons for getting involved. A critic of the war would want to show that the war was fought for unjust, selfish reasons. .
However, all these views do not make clear all the reasons, especially the long term reasons, for the war. US involvement in Vietnam was a result of, at first the Second World War, then the Cold War and the French defeat in Vietnam, especially at Dien Bien Phu in 1954. It was a result of fears about the threat of Communism and the Domino Theory -in particular where Japan was concerned, and the foreign policy of John Foster Dulles; who was responsible for the SEATO treaty. Further involvement came as a result of the disastrous regime of Ngo Dinh Diem. The Gulf of Tonkin Incident was the excuse needed for more direct action in the form of Operation Rolling Thunder, which, when it proved less successful than imagined, led to the deployment of ground troops and the long years of war which followed.
