Has this ever bugged you? Are you sick of the lame old explanations with spray cans and flashlights? Does this law simply not make sense to you? You've come to the right place.
A picture's worth a thousand words. Look at the picture below.
No? Let me try. First, we assume that whatever we're dealing with originates from a point or approximately a point source. Second, we assume that thing distributes itself evenly in all directions, as with gravity fields, electromagnetic fields, ideal sound waves, light from a bulb, etc.
Now, the general idea is, as the distance from the source increases, the intensity of the projection decreases by the square of the distance. The square of the distance AND ! Why? Simple: is the surface area of a sphere. And, an object with a point source whose field of effect is distributed about uniformly will form a sphere around itself. Hence, as the volume of effect expands, its area of effect divides itself evenly about the surface area of the resultant sphere.
One way to practically make sense of this is to consider an electric field. The electric field strength is represented by arrows. Now imagine an electron with four electric field lines evenly distributed about the point charge. If you look at a 2D model, you can see that at just about the surface of the electron, the field lines very close to each other. Now look at a distance much farther away; the lines are much more separated. This indicates a decreasing field intensity.
If you imagine this with a spray can, except the can itself is just a metal ball, then (in a vacuum) as it sprays, the total number of particles will uniformly distribute radially outward in all directions (if we maintain the analogy to electric field lines). Hence, at any given point, the number of particles there will be the total original amount divided by the surface area of the 'sphere' that exists at the radius of separation from the ball . So you would get sprayed four times less if you stand twice as far with respect to an arbitrary point, and nine times less if you stand three times farther from the same point.
Also note that since is constant, when we compare one radius of separation to the other, it applies to both and hence cancels. But it is important to note that this quantity is present for any uniformly distributed field in 3D space, for without it we wouldn't be dividing by the total surface area to find the portion of the original field intensity at that point.
Now practically of course we don't have such 'point' sources. However, many sources behave approximately as such, especially considering that the more round an object is, the more it behaves as so. This is evident from light-bulbs, that emit light in all directions approximately perfectly uniformly. And sound, where air disturbances radiate in all directions. And SPRAY CANS, whose point-sources spray particles from almost a 'portion of the sphere', if you'd like to think of it that way.
In fact, if you have a spherical object, it does behave exactly as a point source. This requires a bit of knowledge from physics (or vector calculus, or great imagination), but that's a topic for another day :)
Here's that same image, for electric fields: