It seemed to Sarah that she was the only person who actually existed. Everyone else was just going along, nothing they did required existence. Sure, they had personality and neuroses, but these could just be like a ... a computer program, just programming preset to respond in a certain way. No one is really making any decisions.
Sometimes Sarah would look at a menu and have no idea what to order. She had to come up with a decision from nothing - no supporting evidence, no aesthetic, no pictures. Everyone else seemed to have a reason for everything.
And if there is a reason for everything, for every single thing, then you don't really have to exist, you just have to function, right until there's a reason to stop functioning, like when a bus hits you.
Existence, however, did not make Sarah feel superior to everyone else - usually. Usually it made her feel like a big dope.
The way Donald was taught, her theory might come out like this: everything is an object except Sarah. Therefore the entire universe must be a computer program written for Sarah, even though she hasn't the slightest notion how to get it to do anything.