Aristotle says that a major cause of injustice is greed—he believes that justice is akin to balance in all things, because there is a limited amount of resources in the world and one person’s gain is another person’s loss.

As for the second part of your question, it’s pretty widely known that human beings are social—we live in groups, we work in groups, and we don’t do well in isolated conditions. This is what Aristotle is referring to when he calls humans "political." Bees are also social creatures, but the difference between humans and bees is that humans share a more complex form of communication—language. We are able to discuss things and debate things with each other, which is why we are the "most political animal." Aristotle doesn’t believe other animals are capable of reasoning in the same way that humans are.