What creates the personality of a dementia patient?

What makes them nice is to let them think they are still independent as much as possible. They want to make decisions and they want to think the good ideas are ones they came up with.
Remember the social filters and restraint are gone. If they don't get what they want, they will try different strategies to win. If sad wins, they are sad. If tantrums win, they throw a fit. If sickness wins, they are on death's door.

In my moms case the negative aspects of her personality - which were always there and always out weighed the positives are greatly amplified. The positives are almost non existent anymore but I see glimmers every now and then. Mom was always very independent and didn't take well to being told what to do - so now that she is completely dependent on others and the NH pretty much dictates her schedule - well, it only exasperates the situation.

I am sure it is a whole mix of things -- what their personalities were like before, the areas of the brain affected by disease, limitations and losses in their lives, pain, dizziness, confusion, caregivers who get on their nerves, ... and the list goes on. :)