In chapter 3, under "Creating an Interface"
The book suggests -
Interfaces are great when establishing dependencies across components to isolate volatility, but they are not needed everywhere.

Please confirm my understanding in trying to map the book example to the above suggestion -

Using "Responsibilities" interface, we are isolating "Teacher" from the implementaion changes ("Current" to "a new class", the volatility). Here, the implementations are the componenets accross which dependency of "Teacher" has to be established.

In which ways the design of giving every class in my system an interface a bad design? I would choose to do this to play safe, because I can't forsee the future where I might have to have an alternate implementations of the class. Should I orient myself to thinking in terms of single/multiple implementations when deciding to employ an interface?

Please suggest me book/articles where deciding on when and WHEN NOT to use Interfaces is discussed.