Well, comparing to Romans both Celts and Germans acted more barbaric, and Romans were the most important nation in building of the Western civilization in the antiquity, i guess that's why the Romans are mostly the good guys. Despite the Germans are mostly illustrated as villains in the documentaries or movies some of German characters have attracted many fans,such as King Cerdic in King Arthur movie History channel has become a joke, since they began to produce more hollywood-like programs and entertain people instead of educating them!
That´s normal, just think about the Europeans in the 19th century. Because they were so high developed and then met primitive cultures nearly everywhere, they even thought their race must be superior XD
True, but racism and nationalism has not much to do with development level, the Mongols were inferior to every single nation they invaded, but still they considered themselves as the master race, of course on their own way !