The eventual consumers of the class expect capabilities in the class that necessitate a complex interface. This makes the class easier to learn, maintain, extend, and use reliably.
Simple interfaces clarify and concentrate the class' responsibilities.
However, some classes are naturally complex, often due to the fact that instances of the class may be used differently by different applications or users, each of whom expects an interface that suits her purpose. In other words, a general class may be used in different, specific contexts. Furthermore, code that uses a general class in one context will not care about methods that exist to support any other context. Yet, interplay between the various roles of the general class exists, thus leading to the tight coupling within the single, general class. For example, a parent's ability to attend a child's recital is not a concern for code that treats a person as an employee.