Beyond his brothers and sisters, his family in Heaven, there were only a handful of people he actually cared deeply about. While the Winchesters were on that list, Dean being higher on that list than Sam, because of the profound bond they once shared, with Meg also being on that list. She may not see it, but it was there. Castiel couldn't claim he cared to that point about a lot of people, but she was certainly someone he wanted to protect with all of his being. And even with his situation with the brothers being shaky at best, he wanted to protect them as well. He didn't want to see something happen to them, not if he couldn't prevent it. And after the way he seemed to let them down over time, he wanted to make up for that as well.
"I know I need to speak with him. And with Sam. But I'm not sure about it." He knew admitting this just admitted that he felt shame for what happened and wanted to avoid the rush of human emotions that came with that, because he was confused as how to sort through them and it only overwhelmed him when it came down to it. "Dean was having a hard time trusting me after that and had to be pushed by Sam to help me in my mission to complete the trials I thought would close Heaven's gates and save my family." Those "trials" turned out to be Metatron's deception, a trick to cast all of his brothers and sisters (and himself) out of Heaven and it was enough to play on his features how he hoped Metatron didn't show up here. Because the scribe had something he wanted. His grace.
"You don't have to leave," he insisted, a furrow of his brow following as he said this. He knew Meg and Dean didn't see eye to eye and never would, but it didn't mean he thought she shouldn't be there.