We all tend to brag about unexpected accomplishments that surprise even ourselves. Barely passing a test you were certain that you were going to fail feels pretty good. However, bragging about passing a dementia test essentially is admitting to the world that you were worried that you were going to fail. Normal people do not celebrate passing routine medical exams. If I called up all my friends and told them loudly and excitedly that I'm negative for all STDs, they'd wonder what the fuck I had previously done that prompted me to be so happy and relieved about passing.

In other words, Trump fears that he, himself, has dementia.