Blake compares the natural tendency to fall into habits in a relationship to nature itself, in that both appear and grow automatically. But nature can be harsh and unpredictable, and even sweep away your expectations. The same applies to a relationship—becoming too engrained in habits can sap satisfaction or create feelings of neglect.
As a result, the “wall” Blake mentions is an emotional one. He calls for a “forest fire” to purge the relationship and start fresh by figuratively burning down the barriers between them.