The issue is not just Iran having the Bomb. It is, among other things, that of a greater Nuclear Arms race in the mideast. The Saudis are much more worried about Iran than even Israel. Also, the leaders of Iran have a much different concept of winning then anyone in the west. The whole 12th Iman plays into their logic. A major Sunni - Shite war is in no one's interest but an Iran bent on re-shaping the sunni - shia playing field especially in IRAQ could create a scenario where this is plausible. So, dealing with IRAN early is necessary.
Israeli soldier lightly wounded after being hit by car south of Hebron (Haaretz)
from the article: U.S. rejects Israeli assessment that Iran sanctions are ineffective