Birds of a feather flock together.

The proverb suggests that people with similar interests, habits, or outlooks tend to associate with one another. It describes a pattern, not a rule that every friendship must follow.

It can also imply that a person's choice of companions says something about their character. In that use, the observation may carry a critical or suspicious tone.

Use it when drawing attention to similarities among people who spend time together. Consider the tone: it may be a neutral observation, but it can sound judgmental when used to assess someone by their companions.