catnip and coffee

There is a highway I travel frequently that is normally pretty quiet. On a typical day you can drive along without encountering very many other vehicles. I can set my cruise control and pretty much keep it there. Why is it then, that there always has to be one or two other drivers that absolutely must pass you, pull directly in front of you, and slow down?? I sort of understand it on a busy highway, but on one with barely another car visible, I just don’t get it.