Intelligent life in the universe has learned that war between worlds is destructive. It is literally prevented from occurring. This tempers the desire for conquest, for such ambitions prove to be a detriment to everyone involved. Should humanity desire conquest in space, other races will amass against our world to such a degree that we can never oppose them.
Stability is more important here than the advancement of one’s empire—stability and the steady flow of resources. Restraint has proven to be beneficial in this regard. For a young, aggressive race such as humanity, this is hard to understand.
Humanity is like a wild adolescent who has great passions, great ambitions and great plans, but is reckless and self-destructive. Humanity is now emerging into a mature environment where such passions and recklessness will not be tolerated.