If it's such a great deal, why does he have to threaten to veto any attempts to oppose it?

BREAKING: Obama says he will veto any legislation that blocks Iran deal.
— The Associated Press (@AP) July 14, 2015
If the Iranian nuclear deal is such a great deal, why does President Obama have to threaten Congress with a veto if they oppose it? If it were such a great deal, wouldn't Congress just pass it?

Oh wait... remember Obamacare... remember all the tricks they used to get it passed -