Humans are intrinsically social creatures. We are designed to operate in tribes. Our ancestors had a direct need for social interaction, as a matter of survival. There is safety in numbers, it makes acquisition and allocation of resources and skills more efficient, and of course we enjoy the companionship of others. Groups are simply stronger than the individual. By working collaboratively, we are able to survive and accomplish much more than we ever could on our own.
With this built-in "wiring" comes a few drawbacks, though. The desire for belonging leads to fear and distrust of outsiders, which leads to other issues like conflict, and when taken to its extreme, warfare. We develop us versus them mentalities, we feel less empathy towards those in the out-group as opposed to those in the in-group.
This is what we must work to avoid. We must be cognizant of our own biases and catch ourselves when we fall into these limiting mindsets. We cannot fall into the trap of making assumptions about others, especially when those assumptions lead to irrational fear (often of the unknown or the unfamiliar). By cultivating awareness, we can avoid "othering" along with the fear that accompanies it.
Another flaw we have is going along with the herd. We assume our groups are correct, and we allow the ideas and practices of the group to dictate our own thoughts and decisions. We don't want to make waves, lest we end up being ostracized from our social networks. This is a natural inclination. Excommunication often meant tremendous risk to our survival. If we no longer have the safety of our group, we must attempt to gain favor with a different group, one which is naturally inclined to distrust us from the outset. But again, we must resist this.