The first attempt to heal Sam had not gone well. This was no broken bone or monster bite. The trials had altered Sam on a level that the boys couldn't even see, and it had been killing him. Castiel was relieved that Sam had not carried the rituals through - id he had, then there really wouldn't have been anything Cas could do for him. At the very least, he would make it so that Sam could live out his life without pain. Sam deserved that, after everything.
Castiel wasted no time, flying to the Winchesters' living room and landing next to his friend on the couch. "Hello Sam," Cas said, watching him with sympathy. "You are not looking well." Even though Cas had already tried to heal the extensive damage, Sam still looked exhausted and pained. Castiel wasn't sure how he was as active as he was, but the Winchesters always were stupidly resilient out of sheer willpower alone.
No matter what Sam said, Castiel knew that he himself was of no consequence here. What else was he, but a fallen angel whose hubris led to massacre of his own siblings, of the humans he had professed to love and protect? Castiel was already straining to manage what he had taken from Sam back at the hospital, but he would heal Sam, no matter the cost. He had to. What value did he have it he couldn't at least fix what was broken? Perhaps he simply had not tried hard enough the first time. Perhaps he'd missed something.