Forced by limiting the space through which the fluid must flow. Think
of your garden hose. If you put your thumb over the end and constrict
the space the water flows faster through the opening. As the speed
increases the pressure decreases, air moves from high pressure to low
pressure and the wing of the airplane is in the way of this movement so
it is lifted up with the high pressure air. This also explains wing tip
vortices and why for a given configuration a higher aspect ratio wing
will produce more lift than a lower aspect ration wing.

PV=nRT