Why can’t Justine Greening pretend to enjoy her job?

Does Justine Greening enjoy being International Development Secretary? I ask only because while the text of her speech to the Conservative party conference contained a lengthy defence of aid spending, the minister managed to sound about as thrilled as someone who had just discovered their bus fare had gone up by 10p when she actually delivered it. If nothing else, the speech demonstrated that Andrew Mitchell was a loss to the case for ring-fencing the aid budget when he made his fateful journey from that department to the whips’ office. Mitchell always launched into an impassioned and compelling explanation of the liberal interventionist case for development spending. Meanwhile Greening is known to disagree with that target: today she hardly sounded like she had caught any passion or conviction for it after a year in the job.