These are generalizations of course, but they do offer a very real aspect of our support for Israel as you ask.
I have seen pro-Israeli US supporters to fall within these categories:
1) The elite financial and industrial crowd support Israel only so far as they can make profit. They know no sovereignty or loyalty outside of profit. These people are not Republican or Democrat, Liberal or Conservative. These are the real owners of our entire system. If tommorrow no money could be made off it, Israel would be no different to than Sri Lanka.
2) The Evangelical Christian crowd supports Israel only in as so far it leads to the return of Jesus and his rule. The Bible says the land belongs to the Jews so that trumps all logic you can present to them. The Elites use this crowd over and over again as financial recruits for the extremists in Israel as well as our military endeavors.
3) The common American. His support comes from a combination of complete ignorance of history, constant Holocaust awareness, and propoganda that the Islamic militants we face are the same that Israel does. Of course, they are completely different, but he does not know that because he has never bothered to learn for himself.
Within these groups the usual suspects of explnanations include the democracy, war on Islamic terror, being Allies, and many other flawed concepts.
Let us be real for a moment. When it comes down to it, I am sorry to say, the partnership with Israel does not bring anything good to the common American. Not one. It never has. I am still waiting for someone to prove me wrong...
First, it does not take a genius to figure out that in an age of nationalism at the end of the 19th century, allowing the foreign colonization of an area on top of a native population desperate for self-determination is only going to create conflict. That has not changed. We supported the plan blindly back then because we were arrogant, bigoted, and ignorant. Just like everyone else, it was the context of the times. What is our excuse now?
The democracy argument is awfully naive since we recognize democratic regimes at our whim, when it serves us we do, and when it does not we do not. Israel is a democracy whose entire spirit of being is based on establishing a Jewish majority using Jewish immigration onto the land. Any party that does not adhere to this general concept is not welcome, which makes it a strange democracy to me. Likewise, we do not give Egypt billions in aid to refurbish the pyramids. It goes to keep our man, Mubarak, in power. So much for promoting true democracy. In the first Iraqi vote after Saddams removal, our congress was considering not recognizing the winning party because the new Shia party did not want to recognize Israel. Finally, if Israel were such a great democracy it would have no need of a two state solution to ensure it remains a Jewish state demographically in the future.
The next argument is the war on terrorism. This is completely void of any logic as well. Israel is not a buffer preventing the Islamic fanatics from arriving on our shores. The fanatics we face are few and drive pick up trucks. They are funded by the Pakistani ISI, which takes our covert funds to arm the militants we face. Yes, we create our own enemies to ensure that those areas remain destabilized enough so our forces can be there to 'fight terrorism'. The militants are always present but never strong enough to do serious damage. It is not a coincidence. As far as Israel and our relationship in the Middle East, how does supporting Israel help moderate Arabs see us as their friends? Quite the contrary, everytime Israel attacks with an Apache, F-16, or American provided weapon it is a reminder to the Arabs that as bad as those fanatics in their own countries are, America is still not to be trusted.
One may say because of the Holocaust we have a moral duty to protect the Jews from it happening again, but according to the Zionist founders, they have always stated that the west would have to guarantee their safety in the Middle East. That was 40 years before the Holocaust. To make matters worse the whole concept of the Modern Israel was to escape persecution. Was Palestine the best place for that!? That is an Israeli problem not an American one.
Apart for the elites making money off of the perpetual war and weapons Israel buys off us (With our own tax dollars of course) I still cannot think of one positive component.