Why were US veterans resented when they returned home from the Vietnam War?
Particularly after Tet in 1968 there was a large anti Vietnam war sentiment among American civilians.
The Vietnam War bitterly divided Americans. Returned soldiers were often challenged by other Americans about their participation even though many were drafted. A soldiers service could be counted against them in employment.
Combined with the lack of fanfare and welcoming by the US Army on their return and the lack of procedures to help with PTSD many soldiers felt let down on their return. Soldiers are often anxious about fitting in on their return home. Many soldiers tried to hide their service experience.
The positive feeling and celebration of the 2nd World War return didn't happen.