Huge financial obligation (we were pumping millions of dollars to aid the French, then slowly military aid as well). The French were fighting to retain control of Indochina (Laos, Cambodia, Vietnam). The people were revolting against years of oppression and the huge gap between the rich (French colonial masters) and poor. The majority of the land was owned by a tiny percentage.
Once the French were basically defeated, they allowed, or gave into a "communist" North Vietnam. Eisenhower didn't like that shit, so the US basically took control of the south and set up a puppet government, sent “advisors”, unleashed psychological warfare against the north. Then Kennedy secretly sent hundreds of troops and advisors, a hundred or two were killed, and then finally Johnson declared war.
The US doesn’t like having peasant uprisings and overthrowing capitalist governments. Makes them nervous. Other countries might soon learn that the capitalist pigs can be defeated. If Indochina collapsed, and Vietnam, then so could the rest of the world (domino theory).
Economic factors, such as the need or want to control the wealth or potential wealth of the region were important factors for US intervention as well (see present day Iraq).
Also plain arrogance on the part of the US ruling class. They ignored the humiliating defeat of the French and figured the French were just fuckups (hadn’t won a war since Napoleon) and it would be a walk in the park for the mighty US military, fighting one of the poorest countries in the world (even though the US fought to a standstill in Korea just a few years prior).