React's default behavior is that when a component re-renders, _everything_ underneath it in the component tree re-renders as well. If you want a component (and its descendants) to skip re-rendering, you can implement the `shouldComponentUpdate` method. You can put any logic you want into `sCU`, but the most common implementation is to compare the contents of `this.props` and `nextProps` to see if anything actually meaningfully changed.

You _can_ do a "deep equality" comparison that recurses through every nested field in both `this.props` and `nextProps`, but that's relatively expensive. The alternative is "shallow equality", which uses pointer/reference equality checks for each field in both objects. However, in order for that to be useful, you need to manage your data in an immutable fashion so that each update results in a new object/array reference, rather than directly modifying the existing objects.

So, you don't _have_ to manage data immutably in React, but doing so enables performance optimizations, and also goes along with React's functional programming influences.