What you’re hearing is part of the constant ambient sound that we don’t often register because it always surrounds us.
The shell reflects and focuses higher frequencies back to our ears. The reason is that high-pitched sounds have short wavelengths, so they tend to bounce when they hit a hard surface.
The longer wavelengths of lower sounds tend to go through surfaces (which is why music through a wall or car window sounds so “boomy”). After bouncing around the shell’s inside walls, the higher tones travel back to the ear randomly, creating a whooshing, echoey sound.
The fact that a shell sort of sounds like rushing water is just a lucky coincidence. To other listeners, it might just as easily sound like wintry winds, ventilator fans, or heavy traffic in the rain.
Of course, there’s nothing special about seashells doing this. You can do the same thing (in stereo yet!) with a coffee mug cupped over each ear.
Not surprisingly, if you’re actually standing on the beach, a fairly noisy place, the “ocean” sound in the shell will be much louder than it would be in your own quiet kitchen.
The same is true if you were standing in a crowded lobby of a building: the louder the environment, the louder the “ocean” sound.