Dean's making some fair points, but Castiel can't shake the feeling that this is the same as playing with fire -- or worse, putting your hand into a pit of vipers.

"And what do you think would happen if the plan didn't work out and Lucifer realized that he'd been used?" One of them will pay for it, that's for certain, and it could very well be Dean. Castiel's been working to keep Dean safe, and allowing him to try and get the best of Lucifer is the equivalent of letting him set his head in place for the guillotine.

Maybe he's overreacted to some degree, but there's no denying that Dean has a suicidal streak and Castiel's been trying to keep an eye on that here. "... If you ever see fit to 'use' him, speak to me about it first." It's not an order, but it is a request, and one that he hopes Dean will agree to.