User blog comment:Gcheung28/A Middle-Earth Character Tree/@comment-5318101-20121222050505/@comment-101.162.9.143-20121223124948

book of unfinished tales actually explaines all that.

Dale was originally their home, but after smaug took it the dwarves fled into exile. now Thorin wants it back and was willing to assemble an army to do so. gandalf however talked him out of it, as smaug would decimate any army.

WHY Smaug attacked the place is a mystery, some thing it was saurons (or his captain - the witch king of angmar who lived close by for some time) doing, Smaug just decided too, or even morgoth melkor somehow has a hand beyond the grave.