Initially, Castiel wanted to reach out and touch Meg, but he was slowly realizing that on some occasions when he got too close to her, it would cause her to withdraw from him. It wasn't a truly deep thought, not something that he allowed himself to dwell on for too long, but it was something that he was beginning to notice and thus, it caused him to try and do something he rarely did. Note personal space. He was used to Dean constantly chastising him about personal space, so it was something he was aware he had an issue with more often than not. Even if the position he was in with Meg was much different than with Dean. The relationships were not the same, but he could only apply his findings as he seemed fitting.
"I thought so, but I've only watched a handful of films and I only meant would the ending be as bittersweet as the ending of Casablanca." That was the only way he could have phrased the ending of Casablanca, because the two leads were in love, yes, but they abandoned that for what they deemed to be a higher purpose, a chance, in their opinions, for Ilsa to be happier, despite how it was hurting them both.
When Meg chose to lay her head in his lap, it caused a slight hint of surprise to play on his features, but it didn't last long. He didn't mind it, it wasn't an invasion of his personal space because she was welcome to lay on him. As proof of this, his hand came to her hair and his fingers gently tangled themselves into the strands as he idly ran them through whilst paying attention to the television screen.