"You'll be the first I ask if I ever need help." That much is true, she's far more likely to come to him if she needs assistance than anyone else she knows, given their history. They've been through a lot together, and he's one of the few who she feels would understand better than most the problems she may face. She doesn't feel like he owes her anything, and she certainly doesn't want him feeling that way, but having Castiel tell her he's willing to return the favour goes far in the way of comfort.
"Well, can you describe what it is that you feel like you want now?" She's accustomed to these desires in a way that he's not, the experience already under her belt. If anyone can recognise it better than him, it's Anna. "I might be able to help you figure it out."
It's all too easy to slip into the role of mentor all over again. It's a new situation, granted, and she certainly never expected that things would turn out this way, but the one thing that hasn't changed is her desire to help him. It's become so important to her to assist him trough this, almost the most important thing to her. She never would have suspected that after endless time as his superior that one day she would be helping him to rebel, but then again she never would have expected then that she herself would one day tear out her grace for a chance of a different life.
"It will take some getting used to, but you'll learn what these desires mean in time."