Israel should not gain membership in NATO because it is not in the North Atlantic sphere of influence. It is an important ally of America, but it is not involved in European politics. NATO was founded to protect Western Europe from the Soviet Union, and it still largely counters the threat of Russian intrusion into Europe.
NATO was created to protect Western Europe from possible Soviet aggression, and was never intended as an alliance that would last for all time. To have Israel join NATO, when Israel is not under threat from the Soviet Union and arguably not under threat from any of its neighbors would be a pointless act.