This question has a very long answer.

So as you probably know, from the first century CE onward, the Roman Empire used the rivers Rhine and Danube as its eastern and northern borders with the Germanic world. At the beginning of this period, Eastern Europe from the Rhine to the Carpathians was populated largely by Germanic peoples who lived in small agricultural communities, most comprised of less than 50 families. These people had few possessions, and social stratification was not intense. Most of the Germanic populations (i think ~60%) were free farmers and women with the rest being slaves. There was little political unity in the Germanic world; tribes were small. Warbands during this period typically numbered in the low thousands and rarely above ten thousand. But over the centuries, prolonged contact with the Empire changed the Germanic world.

The Germanic world's frontier border with the Roman Empire allowed them greater access to wealth from the civilized Mediterranean world than before in addition to increased cultural exchange. More advanced agricultural techniques from the Mediterranean world made it north into Germania and populations grew. Wealth from trade and raiding filtered up at increasing rates. This led to social stratification among the Germanics. More wealthy warlords with the ability to fund larger bands of permanent warriors gained prominence. A new social class of aristocrat warrior was created. Over time these warriors saw more and more the use in unifying in larger groups to more effictively take wealth out of the Roman world. Many of the tribes unified into confederations such as the Franks and Goths. Some in the aftermath of the Hunnic Empire even combined with Iranian speaking steppe nomadic peoples and migrated all the way from the Black Sea to Spain.

Source: Empires and Barbarians by Peter Heather