While I surely will not say that the Israel lobby doesn't matter, it does. When the U.S. votes to prevent sanctions against Israel at the Security Council, it does so for several critical reasons: First is the principle that it will not approve unbalanced resolutions. The Arab-Israeli conflict has long been complicated and the U.S. has understood that only to blame one-side, and in the case of the U.N. it always is Israel, is neither fair nor productive.
Second, the U.S. has had unique influence with both Arabs and Israelis in the region exactly because it doesn't allow for one-sidedness in its views. It is no accident that the progress that has been made over the years - peace treaties between Israel and Egypt and between Israel and Jordan, and talks with the Palestinians - has involved U.S. intervention. Only the U.S. has credibility with Israel, partly because of our Security Council position, to ensure compromise on all sides. And because of that credibility, it is the main interlocutor.
Third, while the U.S. is depicted as pro-Israeli because of UN vetoes among other things, the U.S. should not be seen as anti-Palestinian or anti-Arab. The American view is that this is not a zero sum gain where if one gains the other automatically loses. The U.S. goal is a two-state solution where each people can live in peace and security in their own state. This is therefore both a pro-Israeli and pro-Palestinian perspective, and offers the only hope for the future.
Too often U.N. resolutions reflect a perspective that may be pro-Palestinian, but is also anti-Israeli. This does not help and deserves a U.S. veto.