What does need have to do with it? Do you have friends only because you "need" them? Are you the type of person that walks out on a friend because it is inconvenient?
Israel and the US have a common enemy. That enemy will be our enemy whether we stand by Israel or not. We may not need Israel, but its always nice to have friends, whether needed or not.
It is ridiculous to even try to compare personal relationships to national ones. The federal government is charged by the Constitution to defend the inalienable rights of those living in the U.S., not in foreign nations. The government is supposed to do what's best for U.S. citizens, not Israeli citizens. Look at it this way: if our relationship with Israel helps our government defend the rights of U.S. citizens, keep it up. If, however, such a relationship invites terrorist attacks and unwanted enemies, break it off.
The argument can be made that by posturing ourselves as an unwavering ally of Israel, we have bitten off more than we can chew. Your assumption that we would still be targeted by terrorists whether we stood by Israel or not is completely unfounded.