The Doctor didn't want to admit it, but he was scared. His life had been cut short once more, and this was his favorite body to boot. Though he would regenerate and live to see another day, this body would not. The 11th Doctor would cease to exist and someone else would take his place. He would die, but the Doctor won't. And thus with a tearful smile, he backed away from Clara, not wanting her to suffer from the regeneration process itself.
"Goodbye Clara," he muttered to himself, too soft for her to have heard.