Why did the equality of inertial mass and gravitational mass puzzle Einstein?

The equality of inertial mass and gravitational mass was a source of profound curiosity for Albert Einstein because it explained a "minor miracle" in Newtonian physics: the fact that all objects fall toward Earth with the same acceleration, regardless of their mass or composition.12

In Newtonian mechanics, these two concepts are distinct:
* Inertial mass represents a body's resistance to acceleration when subjected to any force.31
* Gravitational mass determines the strength of the gravitational force experienced by a body within a gravitational field.31

Newton postulated that these two masses were equal, which mathematically ensures that all objects fall at the same rate. However, Einstein found this equality deeply significant rather than merely coincidental. He realized that if a person were in a freely falling elevator, they would feel weightless because they and the elevator are accelerating at the same rate. This led to his equivalence principle, which posits that it is impossible to locally distinguish between the effects of gravity and acceleration. This insight allowed Einstein to reinterpret gravity not as a force, but as the curvature of space-time, where all bodies naturally follow the same geometric paths.142