Obviously they're closely tied politically, with Israel's huge lobbying power. Netanyahu gets these massive standing ovations when he address the US congress. They go on for like 10/15 minutes. Everyone just stands there and applauds. It's bizarre.
There is also a weird religious thing going on. I watched a Louis Theroux documentary on Israel, and American tourists take these guided tours with heavy security around the occupied areas where Israel is expanding illegally. Basically just to show their support for God's chosen people. Young Christian Americans go over there for long periods at a time to help out settlers. Theroux interviews them and they speak about Israel is this incredibly reverent tone.
It's all very weird. There's this strain of something that flows through American society that creates this bond with Israel, but I'm not entirely sure what the origin of it is, or what keeps it going.