While Israel partisans in the US and some others (often former Israel partisans) sometimes claim that the US-Israel "special relationship" is driven by the US, Israeli writers are more honest. Most recently, retired Israeli diplomat Alon Pinkas writes on Ynet* (emphasis added):
Apart from Israel's ability to defend itself, there never was – and probably will never be – a more important strategic asset to the country than its relationship with the United States. Since its founding – and especially since the end of the 60s and the start of the 70s – these ties have provided Israel a superpower to lean on, a supporting pillar of military deterrence, and a force-multiplier in the international arena.
The United States has not only vetoed more than 50 anti-Israel resolutions at the United Nation's Security Council, it has also provided military aid totaling more than $140 billion, as well as access to American weapon systems and advanced ammunition. But the US has also garnered a reputation as an almost-automatic defender of Israel – its layer of protection from international isolation.
The relationship between the two countries has often been defined as "special", "extraordinary", and an "unshakeable alliance." Since the 80s, Israel has often pushed to define itself as a "strategic asset" for the United States and, though Americans have never used the phrase themselves, they have not denied it.**
But on this matter there is some confusion in Israel borne of an exaggerated sense of self-importance. The strategic asset in this equation is the US for Israel, not the other way around.
* Ynetnews, according to its website, "is part of the prominent Yedioth Media Group, which publishes Yedioth Ahronoth – Israel's most widely-read daily newspaper – as well as several popular magazines and dozens of local publications."
** Actually many American experts have denied that Israel is a strategic asset – see, for example, The National Summit to Reassess the US-Israel Special Relationship].