Why the pressure of atmosphere doesn't crush you when you e.g. walk outside? Why the pressure of atmosphere doesn't crush you when you e.g. walk outside? I mean the density of air is $1.26 kg/m^3$, so with $100 km$ above us, it exerts much pressure on you when you walk outside. 
 A: If you were a completely empty shell you'd likely be crushed immediately on finding yourself in the earth's atmosphere. But you are filled with stuff (blood, flesh, bones) which is also at approximately atmospheric pressure. If you consider a point on your skin, the pressure of the air on the outside pushing it in is exactly matched by the pressure of the contents of your body pushing it out. So the net force is zero.
A: To add to Dan's answer - the same thing also applies indoors.
Even with a roof over your head the outside air presses on the windows, doors and walls with the same air pressure, and so compresses the air inside the room to the same pressure.
A: Pressure acts in all directions. it does not care about direction. pressure is a property of the fluid itself. but the way you CALCULATE it is to find the 'weight of the air above'. this does not mean it is literally the weight above.
The reason you don't feel it is because your body is built as a vessel in such a way that there is almost zero NET forces due to pressure acting on your organs.
If you dive into water, the deeper you go, the body now DOES have net forces acting on it. this is how you feel water pressure. if you provide enough force, the body will contort and change to then equalise those forces back into equilibrium. this will probably be what kills you
