He doesn't understand why he chats cheerfully to Cloud. The man has been unresponsive since Zack pulled him out of that tube. The most Zack sees and hears from Cloud is the fluttering of eyelids or the grunts of unintelligible, garbled words.
Either way, as he brings Cloud along, he speaks.
He talks about everything and nothing. Slowly, as he runs out of topics, he starts asking questions he knows Cloud wouldn't be able to answer. Without expecting an answer, he asks whether Cloud knows of whom Tifa is talking about when she mentions the blond SOLDIER.
Eventually, he runs out of things to say as they drive around aimlessly.
"You know Cloud," Zack finally says. "Your email - I'm sorry I didn't say yes sooner. I would have liked to meet your mother."
Cloud stares blankly at the floor, leaving Zack in the silence.