Germanic culture was no more warlike than their Roman counterpart. In fact, if there ever was a culture, people or civilization that loved to conquer and plunder, they were the Romans themselves having effectively enslaved or subjugated the entire “known-world”. However, the Germanic peoples had no tradition of politically structured societies which creates a certain image of chaos and destruction that is probably based in reality as violence was, more often than not, the manner in which feuds and disputes were settled among them. It’s not debatable that when the Germans crossed the Rhine in 406 they raped, enslaved, stole and killed, effectively ending civilization in Gaul and beyond but it is debatable why so many Germanic tribes wanted to cross the Rhine. Unguarded frontiers, frozen Rhine, tribal wars resulting in famine combined with bad weather or the dreaded Huns could all be reason why such a massive migration took place. One thing, at least to me, is clear, the Germans would not have migrated in such large numbers if the land where they came from was safe and economically viable. I don’t think so many different tribes, many of them rivals and engaged in war between themselves would have taken such a massive undertaking. It was, simply, a struggle for resources.

When colonialism started in Europe and eventually became a trend there was without the shadow of a doubt a struggle for resources and supremacy in Europe. The eastern land trade routes into Europe were quickly drying up and there was heavy infighting between European powers constantly trying to get the upper hand on the other. There was also an economic crisis that was occurring in Europe in the 15th century called the Great Bullion Famine. It was, very succinctly explained a major shortage of precious metals used in currency like gold and silver that facilitated trade with the Middle East and Asia. This shortage happened because there was not enough gold available to be mined in Europe and there was massive trade deficit with the East (Middle East and by extension India) because, unfortunately, Europe did not produce items of great value like silk and spices and the manufacturing industry was still relatively behind compared to their eastern counterparts. The discovery of African and New World gold and silver along with direct trade route with the Eastern Indies by sea provided the Europeans immense wealth and fixed their stagnated economies. Overpopulation in Europe was also becoming a problem that can have potentially disastrous consequences due to increasing social pressures and colonization efforts greatly alleviated this trouble.

However I do think that there are some sociological reasons for colonization to be performed almost exclusively by Europeans. For instance the Chinese did not colonize and were very isolationist for most of it’s history because “it wanted cultural and political hegemony over its neighbors who would acknowledge it as the paragon of civilization and pay it vassalage. It didn't need colonies economically, nor did it need them to relieve social pressures.” This quote come from another post that asks “Why didn't China join the rush to set up colonies in the New World?” and I think reading it will provide you some key insights to understand the motivations and stimuli that led to colonization happen the way it did. One thing is clear though, a civilization that asserts it’s cultural and political strength upon their neighbors can only do it if it’s already economically self sufficient and stable.In the 15th century, Europe was not politically, economically or even culturally stable.

I could go into much more detail on why colonization was performed almost exclusively by “”“Germanic Europeans””” as it’s a very complex subject but I would not give much merit to the idea that Germanic “warlike” culture would be a reason for this. I can see why you and many other people have made this connection but I simply do not see any truth to it.

"Why didn't China join the rush to set up colonies in the New World?" : https://www.reddit.com/r/AskHistorians/comments/4g61y0/why_didnt_china_join_the_rush_to_set_up_colonies/ by u/omegasavant

Source for most of this: Heather, Peter. "Why Did the Barbarian Cross the Rhine?" , The Rise of the West: A History of the Human Community by William H. McNeill, Béla Petheö, Legal History, Rey Juan Carlos University, Madrid, Spain