Animals are far
more attracted to break into attics where they scent other animals.
The primary goal of rodents is to reproduce. This drives these
critters to constantly look for groups to join. When a rat scents
other rat droppings in an attic, it will try far harder to chew its way
in than it would for a cleaned out attic.
This is the primary reason why animal problems tend to recur in the
same houses over and over again.