Sam could tell them the unfortunate truth in finding out if Cas would like being human is for him to experience it more. Dean can ask Sam to help with this though. Castiel can only go with his gut and believe he will be happy. Castiel searches for his own feelings about that. "It's fine if you do." The situation doesn't seem all that bad no matter which way it lands. For Lucy, he could be human.

"I need you to be okay with it if it happens. I believe I will be okay with it when it happens. It happened before, remember, and there is much less to struggle with here than there was a home." Food, water, income... all of that is provided. Castiel takes joy in being able to be open to human experiences and feel emotions at a much higher rate with a human soul than what angel grace allots him.