Answer:
France and the Holy Roman Empire had geographical boundaries that are very easy to cross: they were rivers, streams, plains, or small forests. Only in the Southeast, the Alps represented a strong geographical boundary.
Beside, both regions had always been among the most densely populated in Europe, and this naturally led to conflicts due to competition for land and resources.
Finally, during most of their history, both France and the Holy Roman Empire were very unstable, being comprised by countless local lordships. This led to internal strife and external upheaval.
