Number of reasons:
1) History. Us Germans tried to exterminate all Jews (and thank God we failed), while the USA and Great Britain, the colonial power of Palestine, fought against Germany. It made sense that after WW2, Israel was founded as a "safe haven" for Jewish life, backed by USA/UK.
2) Israel is the only real democracy in the entire region. Yes, Netanyahu has attacked democratic institution and basic democratic decency, but still: Israel is and likely will remain a democracy, a democracy allied to the US right in the region where the biggest oil reserves are.
3) While Israel does have their own weapons program, they are a good customer of US arms. Some of their budget is actually paid by US Congress, but well, as long as the end result is that Israel uses US weapons and the people employed in the districts of the Congressmen approving these deals keep their jobs, there is no real problem.
4) Intelligence service cooperation. The Mossad is one of the best intelligence services there is, rivaled IMO only by NSA/CIA.