Birds of a Feather Flock Together

Meaning:
Those with similar interests or of the same kind tend to form groups.

Example:
A: John and James seem to get along really well
B: Well, birds of a feather flock together. They both love reading comics.

Did you know?
In nature, birds of the same species often flock together as a safety precaution, since congregating in large groups would reduce their risk of predatation. This behavior of birds is where this idiom originates from.

Reference: http://www.phrases.org.uk/meanings/birds-of-a-feather-flock-together.html