So if i have understood this well light is a constance to all observers no matter the difference in their frame of reference, but for example motion is not if you're travelling on a train and you throw a ball to an observer a the ball would look as though it was traveling the speed it was in the reference frame of the train and the speed at which the actual reference frame was traveling itself... i may have not fully understood this, it could be a very silly question but...
What makes light so special, do we know why it occurs or just know that it (should) occur?