 Why does Ben leave Laura?
The Academy, with its great halls and classrooms and laboratories, lay hollow and silent, an exhausted thing at sleep after spawning its first-born. Just one, which I'd sent to Charlie Taggart. I remembered how, as a kid, I'd pestered him in the Long Island Spaceport, tagging after him like a puppy, and how he'd grown to like me until he became father, mother, and buddy all in one to me. And you, Laura, were with him. "Ben," he called, "don't forget that offer. A moment later Mickey said, frowning, "What was he talking about, Ben? You kept looking at me until I had to ask: "What are you thinking, Laura?"
 Ben leaves Laura because the call to explore the universe is irresistible.
 Ben leaves Laura because he feels guilty that he dragged Charlie to Mickey and Laura's parents.  Ben leaves Laura because she wants kids, and he doesn't.  Ben leaves Laura because the call to explore the universe is irresistible.  Ben leaves Laura because he knows he'll grow to resent her if he stays.