Imagine the code as a graph with nodes and edges. The nodes should be grouped in a way that when you display the graph with grouped nodes, you see few edges between groups. Removing a group means that you need to cut maybe 3 edges, not 30. I.e. you don't want something where every component has a line to every other component.

Also when working on a feature - modifying / adding / removing, ideally you want to only look at an isolated group, with minimal links to the rest of the code.