Imperialism Thoughts

I learned about the reasons why certain countries had to drive to take over a lot of land from other countries. They wanted their resources, land, territories for their growing population, be powerful, and get greedy. Mainly all for economic reasons. The question I would have would be how the countries exercising imperial power are constantly wanting more and more land and taking over other countries. Why aren't they stopping and allowing for other countries to build to make allies with?