Skulls appear to smile primarily because of the exposed teeth and the natural shape of the maxilla (upper jaw) and mandible (lower jaw).
Here's a more detailed explanation:
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Exposed Teeth: The most obvious reason is the presence of teeth. When the soft tissues of the face decompose, the teeth become visible. The arrangement and shape of the teeth contribute to the perception of a smile or grin.
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Shape of the Jaws: The skeletal structure of the face, particularly the curvature of the upper and lower jaws, plays a crucial role. The maxilla naturally arches, and when combined with the upward curve of the mandible, it can create an impression of a smile.
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Absence of Facial Muscles: In life, facial muscles control expressions. Without these muscles, the underlying bone structure dictates the "expression." The skull's inherent structure lends itself to a grinning appearance.
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Human Perception: Our brains are wired to recognize faces and interpret expressions, even in inanimate objects (a phenomenon known as pareidolia). Therefore, we tend to perceive a smile or grin when we see certain facial features arranged in a particular way, even on a skull.
In summary, the "smile" of a skull is a combination of skeletal structure, the visibility of teeth, and our brain's tendency to interpret facial expressions based on limited visual cues. It's not an actual expression of happiness or amusement, but rather a result of how we perceive the underlying bone structure.
