He could feel pride at the almost deceitful reasoning behind Jack's choice. Who would Dean and Sam trust more than Castiel? It's clever. It should indicate that there is some bit of Lucifer inherently present in the nephilim.

But it really doesn't seem like that's what's driving Jack if he already made some pitiful remark about having to lie to them.

"He stole you before you were born!" Lucifer shouts. "He was 'there for you' because he never even let me have an opportunity!"