He loves them too much to stay
Lane Beaumont has always loved Amanda Hawkins. If his life weren't such a mess he'd want much more than their current on-again, off-again relationship. But Amanda deserves a better life than he can offer. So when she gives birth to their son, Lane does the right thing and walks away.
Amanda should be devastated. Except his reaction doesn't make sense. The Lane she knows would never turn his back on her or his responsibilities. Plus, she saw that cowboy's heart melt when he held their son. Something else is standing in the way of their happiness and she won't stop until she finds out what.