"Fine. What do you want while we're out? If you don't choose quick, Sam gets to choose and you'll have to deal with what surprise he brings back." Dean tells her. He sure wasn't keen on letting her be in the motel by herself but if the wards do their job, she would be fine as long as Cas answers like he sure hopes he does. He also had come to learn to have some faith in teens when it comes to being by themselves and though he had gained this lesson from his short domestic lifestyle, his hunter/brother instincts were screaming at him to just stay with her and let Sam get their dinner. But this was a step that would show that she could at least handle herself without them for a short time. She was taught the protocol in such situations while at Bobby's. Better as any to be put to the test. 

Though Dean was being quite serious about letting Sam choose her dinner if she didn't have a decision. She could figure out if he was kidding or not though.