Taking literally, birds of a feather flock together to protect and defend themselves against possible predators, since it’s easier to fend off vultures through the power of superior numbers.
Figuratively speaking, we tend to seek out and be attracted to those who have similar qualities to ourselves. Think of having the same ethics, values, likes and dislikes, and similar beliefs. This even holds true on a physical level, as beautiful people are inclined to be drawn to other good-looking individuals.
We enjoy those who share similar beliefs, and lean to dislike others who have opinions that conflict with our current belief system. That’s because these differences are typically perceived as a personal attack on our character.