It was one thing, watching your companion be ripped apart.
One very terrible thing, mind you, but one thing. The Doctor stayed prone on the ground, the sound of Jack's death screams ringing in his ears as that thing, whatever it was, tore him into several unpleasant pieces. It reminded him of the Year That Wasn't, of Jack's screams while the Master tortured him and the Doctor's frail body keeping him from helping. That was torture, far more brutal than anything the Master's tools could produce.
Once the loud stomps of the creature faded away, the Doctor struggled to get to his feet and limped to the place where Jack had been.
It was another thing, having to find his body for it to regrow.
It took some time to find his upper torso, limp and lifeless. It didn't take too terribly long to drag said upper torso to a safe, empty cave not far from the forest's edge (after all, what Jack no longer had in height, he also lost in weight. It didn't take long for time to start snapping around him and his body to start to regrow.
That was something else all together. Muscle and bone formed out of nothing, and while Jack wasn't coherent, he was still alive, screaming and thrashing as he reformed. The Doctor pressed his fingertips to Jack's temple and tried to take away the pain, but when that failed, he pressed his mind into a quiet, comatose state.
While Jack repaired, the Doctor covered him with his coat and sat, waiting. For all that they'd fought, for all that the Doctor swore he'd never want Jack back on the TARDIS again, he did care about him. He wanted him happy, even if he wasn't certain he could handle having him so close. Jack was willing to die for the Doctor, and this was just another example of how he could.
But the Doctor wouldn't leave. Not this time.