Wikipedia has some very interesting summaries on brain injuries. The introductory section for Acquired Brain Injury has one of the most oversimplified summaries as to why this stuff can be so scary . . .
While research has demonstrated that thinking and behavior may be altered in virtually all forms of ABI, brain injury is itself a very complex phenomenon having dramatically varied effects. No two persons can expect the same outcome or resulting difficulties. The brain controls every part of human life: physical, intellectual, behavioral, social and emotional. When the brain is damaged, some part of a person's life will be adversely affected.