An analogy for a dementia brain regarding memory problems, is that it is like a wobbly book case. The top books fall first. It is the top where the latest memories are so they are the ones lost.
The part of the brain called the hippocampus is responsible for facts, names, biographies and dates. In a brain affected by dementia the hippocampus is like a flimsy book shelf therefore the information that it is responsible for is affected by the dementia.
The part of the brain called the amygdala is responsible for emotion. In a dementia Brian this remains a sturdy book case. This means a person with dementia may not remember a day out but the happiness remains.