score:7
There are many factors:
Keep in mind that this is New Imperialism, which came after Old Imperialism. Old Imperialistic motives can be summed up quickly:
Upvote:3
Note: this is an opinion question (especially the second part) and probably off-topic here. I'll try to answer it nevertheless. I'm not sure there is a single rationale behind this.
One can easily find more rationales. In the end, the psychological factors causing countries to compete didn't go away, they are still there. The important difference now might be the globalization: with the economies of all countries interconnected a war between first world countries would be devastating for all parties involved and cannot be rationalized no matter how hard one tries. So the competition shifted into other areas like economy.
Upvote:7
I think there's a selection bias at play. A ruler that did not care about acquiring more land would typically end up avoiding conflict with the neighbors, and would tend to not be noticed historically. Just because the historically notable rulers were a certain way does not mean that the average rulers were the same way.