Classitis is a syndrome where programmers value writing more classes over writing deep classes.

Having many smaller classes creates more complexity for the overall system because the reader has to read all the small classes to form an understanding of how the system works.

Having a single deep class with more functionality could help to reduce overall complexity instead. The single deep class can hide unnecessary information from the user and provide more functionality.

This happens because we sometimes take refactoring to the extreme and create small classes needlessly.