Because Israel said so.
And Israel's justification is that Iran has been calling for the destruction of Israel. And, now that I think of it,.. many other nations have called for the destruction of America (nations that already have nuclear weapons),.. and yet nothing has happened. So, the saber rattles don't have much merit. I've said this before:
you just don't launch a nuke and call it day - this isn't WW2, where one side had a powerful weapon. World leaders with nuclear capabilities know this and plan/rule/lead accordingly.
Now, the truth of the matter is that Iran is heavily involved in funding terrorists. And, Israel feels that when the 1st nuclear bomb comes rolling off the assembly line, Iran will hand it over to the terrorists that head off into Palestine to attack and kill Israelis. Or, launch and blame the terrorists.
The reality is that no one is THAT crazy. No matter how Byzantine-esque a nuclear strike on Israel from Iran is, Iran wouldn't be able to cover it's tracks, nor escape retaliation.
Nonetheless, Israel has taken a stance that is pretty antagonistic:
- blocking all attempts by Iran to even obtain nuclear energy
- assassinating Iran's scientists (
http://en.wikipedia.org/wiki/List_of_Israeli_assassinations)
And, it is to accomplish one thing; wipe out the Palestinians. In order to do that, they want to cut off any potential attempts by Iran to give the terrorists anything that gives them an edge, since it will benefit the Palestinians.
Israel has had a very aggressive stance against the Palestinians from day one. Israel's strategy is detailed and 'exposed' by Miko Peled;
http://en.wikipedia.org/wiki/Miko_Peled &
https://www.youtube.com/watch?v=CtQKr6f_Uwk
So, in Israel's view:
1) The Palestinian lands are theirs, not ifs, ands or buts. Israel never wanted peace and coexistence. The latter was recently revealed.
2) Anything that can support or strengthen the Palestinians must be destroyed - ASAP. Which is why Iran is getting it's scientists killed, cornered by UN sanctions and a stern 'fuck off' from America's conservative leaders.