Love you... Just kidding

"I miss you," I told him. He gave me a grin and said, "me too". I know he's kidding but he thinks I am too. And how do I say that I'm not?

How do you tell someone you've been fun-flirting with that you're no longer joking? That every day that I see his empty chair is a sliver of ache in my chest? How do I let him know that his smile plays on my mind before I go to bed every night, and his laugh echoes in my thoughts when I'm lying there restless and unable to sleep? How do I make it clear that when I say "I can't live without you", I'm expressing what's in my heart?

But more importantly, should I? Or should I carry on tossing out "Love you, darling!" with a cheeky grin that says, I'm just kidding...