Being with Meg, Castiel had so much more to smile about. It wasn't as if Castiel were never happy and didn't have reason to smile, but with her these things came easier. While, yes, her leaving him after they made love and he told her he loved her was quite a blow to him, it didn't stop him from holding those feelings for her. They remained, unchanged, and would remain so until the end of his days. And that was why he asked the clerk to assist him in whatever way she could, because he wanted to get all the items Meg requested and to do so in a manner that would please her the most. He'd failed when it came to getting the provisions the bunker needed, when he was attempting to make up for his transgression against Dean, to make the brothers trust him and include him again, so he refused to let that happen again with the task he'd been given by Meg.
"Oh. That wasn't necessary, Meg." He told her with that soft smile he held for her alone. "I don't require that. You being here, it is enough." Knowing she wanted to be here, wasn't here because she felt she had to, to take care of him and help him through being human. That was what made up her leaving him, at least to serve as a salve on his heart, to remind him that she did come back and he still had her in his life, rather than lost her to his inability to know when he shouldn't push, because these feelings were new and they were both trying to adjust to them.
"Thank you," he said after a moment, to let her know that he wasn't brushing off the gesture. This was different from when she cooked, previously, because it wasn't in a moment where he desperately required sustenance. "I hope that the items you asked me to get are up to your standards, since you prepared this dinner for us."