Shannon recalls the Doctor saying that each struggle will have its own unique horror. Many factors play into the will to live. It is a combination of the past, present, and future.

The past has a lot to do with the person's proven ability to cope ---to rise above.  The person's present condition also has a lot to do with it. Each day we can discount our past successes and throw no light on the future ---by our mere frame of mind.  But that's the scary part.   In this coma-like state there is no registered mind at all.  And attempting to get the mind back is one of the biggest risks of all ---trying to bring the mind back to where it was before it had slipped away.