I suspect Jesus sighed as the HolySpirit was grieved by the foolish, blindness of the question. Jesus asks why they want a sign. do they want to believe? If they did the signs He gave in His healings, miracles and exorcisms were more than sufficient.<p> He says in this place there will be no sign given to the Jews of that generation (of that persuasion). They were all doomed for rejecting the signs that He performed. <p> In another place He shares that the sign of the prophet Jonah who was 3 days and nights in the belly of the great fish before being spewed out is the sign of all signs.