If people like to play tennis, you will always find them on the tennis court every day. If they like reading you will find them in the library. If they like cricket you will find them on the cricket field. It is only natural that they will congregate at the place where they find their common activity or pursuit as part of their daily activity.They would like to discuss the adventures of their experiences. They naturally like to be in the company of people who have the same interest as they have.
You will notice with birds too and therefore the proverb has been coined