Why do object-oriented programmers usually specify that their data fields will have private access?

What will be an ideal response?

In object-oriented design, usually you do not want any outside programs or methods to alter your class's data fields unless you have control over the process. For example, you might design a class that performs a complicated statistical analysis on some data and stores the result. You would not want others to be able to alter your carefully crafted result. As another example, you might design a class from which others can create an innovative and useful GUI screen object. In this case you would not want others altering the dimensions of your artistic design.
To prevent outsiders from changing your data fields in ways you do not endorse, you force other programs and methods to use a method that is part of the class.