What does this mean if I'm laughing and he's laughing and then he instantly becomes serious?

Answer 1

If this is an on-going pattern with him, I'd be very uncomfortable. I'd become uncomfortable laughing around him; I'd end up feeling like a fool a lot, or like I had missed something important or like I offended him. That would grow old fast. Most important, it would feel to me as if he is trying to control my emotions, rather than just being free to enjoy a moment. You could try gently challenging him when he does this, showing that you can enjoy a laugh without feeling restricted. If you can say this without any churning in your gut, you could try: "Tell you what. You laugh with me this time, instead of mefrowning with you! What do you say?"

Answer 2

It could mean that he was trying to cover up his true feelings with laughter, but then felt the need to address something serious that was bothering him. This sudden shift in behavior could indicate that he is struggling with conflicting emotions or thoughts.