Richard Matheson said this quote

He kept thinking about Mary. What a fool he'd been to let her go. To think, with the thoughtless assurance of youth, that the world was replete with endless possibilities. He'd thought it a mistake to choose so early in life and embrace the present good. He'd been a great one for looking for greener pastures. He'd kept looking until all his pastures were brown with time. (Old Haunts)