The brain is so complex it is bound to fail. It should have many “pieces” that are in low supply or do not function as designed. A brain is a part of human anatomy and we are prone to breaks and sprains and wearing out. It is the seat of thinking and feeling and it can become diseased. With hundreds of billions of connections through synapses our neurons can’t possibly function perfectly all of the time. If I cut off my foot, I can still think and feel and function. If I cut off my brain, that’s it.