R’ Chaim of Volozhin asks why a person who is careful not to sin is call a “Yerei Shamayim” – someone who fears Heaven?

Such a person fears the awesome damage that sins cause to the spiritual worlds “above” ours.  Of course, sins adversely affect the physical world directly, but greater damage is done in Heaven – the spiritual realms, and everything that happens there redounds “downwards.”  He who is careful not to sin fears for Heaven. ~ Seifer Taleli Oros