Why do some Americans vote for the president of their choice the criticise everything he does?

Bush and Obama are prime examples . I ,as a Brit can understand some problems with George Bush who took America into 2 wars and finished his "reign" with a bad recession and increasing unemployment but what has Obama done that deserves all the criticism ?. He even managed to give his fellow Americans a health care system although a watered down one .In my humble opinion he deserves the full support of the American people. Without that the only way for the US is downhill. America should show the world that its a caring, truthfull nation and stop its internal bickering .