This JIRA proposes a feature. I'm willing to try to contribute it given a modicum of encouragement and guidance.
Over at CXF, we get many questions from users who are completely confounded by the complex dependency graph that results from our many dependencies and their many dependencies. I think that they would be less confused by far if maven gave them a tiny bit of help.
The first part of the idea requires an addition to the core POM, which is why I'm starting with a JIRA here. I propose to add an 'explanation' element to the dependency element. This would contain a human-readable string explaining why the dependency is here.
The second part is a goal that I would propose to call 'dependency-map'. This would produce a formatted map of the dependency tree – enriched, of course, by the comments in the first part.