Iran and nukes

Why shouldn't Iran be allowed to do the nuclear thing if they want?
It makes NO sense to Me, you have Israel, who completely deny even having nukes, doing whatever the hell they want and no one DARES say anything against them for fear of being accused of being a Nazi, then you have India and Pakistan at eachothers throats, BOTH armed with nukes, so WHY the big deal about Iran even enriching uranium and not even necessarily building weapons, I don't get it.