Paint Me Not

Anyone know of reasons why, on the same block, some buildings seem to always get graffitied and others don't? I mean besides obvious things like well-lit storefronts or gates. If two buildings are both equally accessible why is one untouched? I see this all the time - do those shop owners have some other tie to the neighborhood? Is graffiti a one-upsmanship thing so a building that gets tagged is more likely to be tagged subsequently by others?