When objects look like each other, people perceive them as having a meaningful connection. I.e. if things look the same, people interpret them as being in a group together.
This might seem like… duh. But in all reality, when you think about it, just because two things look similar does not mean that they have any kind of deeper connection at all. And it certainly doesn’t mean that they’re in a group together.
But, that’s the way we perceive it. In the following examples, all the circles are just circles—the same size, the same shape, and with the same amount of space between them—but you’ll automatically group them by color. Your mind will make groups where there really aren’t.