Why can't we all just get along?

One word. Rules. In a world with such a large population it becomes increasingly difficult to keep everyone playing nice with one another. This is not to say that you need a new law for every thing you think someone may do to someone else. It does mean that everyone needs to play by the same rules. This is made simpler by keeping the rules simple. People instinctively adapt to whatever system they are placed in. If you create rules that promote fair-play, then you will get fair-play. If you create a system that benefits one group to the detriment of another, then the people benefiting will defend the system and the people oppressed by the system will first try to limit how much the other group benefits from the system and then try to dismantle the system entirely. Right now we have a system where the government has grown beyond its intended limits.