Susan's brain doesn't work properly anymore.
After dinner Susan and her little sister were sitting on the couch rubbing their bloated bellies separated by Lucy, the lucky pit bull. Lucy wasn't rubbing her belly, she was licking her lady parts until Susan nudged her to knock it off. Immediately after Susan let Lucy lick her face. Lick her face!

Susan didn't even realize there was anything wrong with that until her little sister looked horrified and said Ew, I'm not kissing you goodnight. How could Susan's brain forget from one second to the next where Lucy's tongue had been?

Susan's never let any other dog get anywhere near her face but she's made exceptions with the new dog because she still misses the old dog.
This doesn't negate the fact that Susan's brain doesn't work properly anymore.