I believe it is primarily religious and political. Although America states we believe in a separation of (church and state) no one is likely to get elected to the office of president of the United States by claiming to be an atheist or for that matter a practicing muslim or buddist.
"Unoffically" the offical religion of America is "Christianity". According to the bible Jesus was a jew and the jews are God's chosen people. It stands to reason a believer/follower of biblical teachings would be supportive of the "holy land". Although most jewish people do not believe that Jesus was "the son of god" there is still that shared belief of the old testament between both countries.
Polictically/economically the U.S. wants a strong ally in the region to assist us in protecting our national interest (oil) in the event the area becomes unstable. America is also supportive and protective of the "underdog". Israel is surrounded by countries that hate it. In addition the Holocaust in Europe naturally elicits sympathetic tones from the U.S.