Social psychologist Karen E. Dill-Shackleford begins with defining what can be considered “a perfect fit.” “A container is ‘right’ for the object(s) it holds when it conforms to the shape and there is no wasted space.” She then gives a simple everyday example: “A bun is the ‘right’ container for a hot dog because it was made to fit.”

There is a difference between seeing the aforementioned hot dog fit into its bun and a cat that nestles in its new bed to perfectly fit its shape. We expect the hot dog to fit – that’s what they’re made for. It makes sense, it’s logical.

Other times, like when non-food related items fit into a sandwich bag, we get satisfaction from efficiently used space. It’s the visual representation of finishing one’s plate. We like its feature of effectiveness.

Sometimes the reason can be pure aesthetics: “There is beauty in forms that seem made for each other,” says Dr. Shackleford. Just like we love looking at the sun setting into the ocean, we can enjoy things purely because of their aesthetic value – it just looks pretty.

If we measure the cat or the pet bed prior to buying, we can say that it was planned. Then the satisfaction comes from it being custom-made to fit the cat. Still, it is expected, the perfect fit is not suddenly sprung upon us.

So here comes the biggest difference in what perfect fits really tickle our brain in the right way and which are just mildly satisfying. The factor of surprise plays quite a significant part in the amount of joy we get.

Belgian experimental psychologist Johan Wagemans has done research on this phenomenon. According to him, there are two explanations of why we love looking at things that fit into other things. Firstly, it satisfies our need for order. In a world where we control very little, it’s nice to see little things that bring order.

Another reason is finding order where we least expect it. The sense of surprise deepens our satisfaction in cases where we expected failure. Wagemans explained this sensation to the Atlantic: “Suddenly you see things in a different relationship and it challenges your expectations about how things relate to each other.”

“The unusualness of the combination might grab the attention, but it’s also how extreme the coincidence of a perfect fit is. If you look at it statistically, and in terms of how these objects are made, and how they came together, it’s almost too good to be coincidental. If it would fit only 75 or 85 percent, it wouldn’t be fun,” Wagemans says.

Dr. Shackleford agrees with the statistics argument: “You feel that good fortune has befallen you because you have defied the odds. Surely, there are low odds that of your haphazard assembly of things, any container will be the perfect fit for any object you possess.”

Our expert also relates this to the psychology of human relationships. “This may mirror the feeling of gratitude we feel when a person meets another person that feels like a custom fit,” Dr. Shackleford adds.

“In the haphazard collection of humanity, the odds seem low that any two would be a custom fit for each other. When you feel you’ve found one, you realize how fortunate you are.”

Dr. Karen also carefully speculates there’s a Freudian angle to liking perfect fits. “Maybe it relates to something biological or even sexual – that body parts that were made to fit together are valuable,” she says. 

“To be explicit, [male and female genitals] need to form a good fit in order for humans to reproduce. So maybe we look for those patterns of fit in the world based on a primal desire for a good fit,” Dr. Shackleford suggests.