While the reason for US hostility against communism is something I will leave for someone else to answer, I can definitely answer why the United States chose to escalate their presence in South Vietnam.
These points are taken from a speech that US president Lyndon B. Johnson held at John Hopkin's University in April 7, 1965.
The United States was there to protect and uphold South Vietnam's democracy and independence. "Our objective is the independence of South Viet-Nam, and its freedom from attack. We want nothing for ourselves--only that the people of South Viet-Nam be allowed to guide their own country in their own way."
They were there to fight against communism and prevent communism from spreading throughout South-East Asia. "We are also there because there are great stakes in the balance. Let no one think for a moment that retreat from Viet-Nam would bring an end to conflict. The battle would be renewed in one country and then another. The central lesson of our time is that the appetite of aggression is never satisfied. To withdraw from one battlefield means only to prepare for the next."
The United States wanted to be seen as a reliable ally. If the US had left South Vietnam to its own devices and being run over by North Vietnam, what kind of ally would they have been? To put it simply: their credibility was at stake in LBJ's opinion. "There are those who wonder why we have a responsibility there. Well, we have it there for the same reason that we have a responsibility for the defense of Europe. World War II was fought in both Europe and Asia, and when it ended we found ourselves with continued responsibility for the defense of freedom."
The invention in Vietnam has to be seen in the context of the Cold War. The United States had been involved in Vietnam since 1950, supporting the French armed forces with money and equipment at first and then taking a more active role after the French retreat in the newly formed South Vietnam. The US leadership genuinely believed that if South Vietnam fell to North Vietnam, then communism would spread to South-East Asia and that countries would fall in quick succession - like dominoes. But if the US could stop the tide by containing communism before it spread, then they were doing a favor to the whole "free world" in their eyes.
It had a lot to do with 'containment.' In the "Rimland" concept proposed by N. J Spykman, a force seeking to control Eurasia is better off holding the outer territories rather than the central area (Heartland concept). Vietnam certainly fell into this concept in a few ways. One of which being its raw geographic location. But American logic in the war also came from a bit of a misconception on Communism. They viewed the 'Red Menace' as a monolith. A massive, soviet-central network. This network posed a serious threat to American interests in the rather chaotic postwar world, and challenged America's ability to shape world order. Particularly when they were absolutely terrified of the massive Soviet presence in Europe (and their nuclear weapons). They were also afraid of China, whom they assumed had major assistance planned for North Vietnam. (In reality, Mao was very adverse to helping the North Vietnamese, and would even wage war against them in the years after the American withdrawal.) Mao Zedong, who claimed that America was an irreconcilable imperialist enemy, also claimed some pretty intense hawkish stances. Famously, he scared the life out of both the Soviets and the Americans by claiming his country was immune to nuclear weapons, as there were just too many Chinese to be wiped out. In reality, the Americans weren't quite right about their views on Communism until Soviet-Sino violence became a weekly occurrence. Communism was far from a monolith, and perhaps about as fragmented as the States were in the years leading up to the Civil War. This, of course, is compounded by reasoning outlined in other comments.
According to Bruce Schulman's Lyndon B. Johnson and American liberalism : a brief biography with documents, both LBJ and Kennedy viewed the loss of Vietnam as both a victory for communism, and a political grenade for the Democratic Party. The Dems blamed the loss of China to communism for the party's election losses to Republican Party during the Eisenhower years. Dems feared that if the U.S. lost Vietnam, Republicans would recapture majorities in both houses and the presidency. Eric Goldman's The crucial decade : America, 1945-1955 was written in the 1960s, but it also makes mention of the political repercussions facing the Democratic Party if it lost Vietnam.