When a wing flows through air, if the air strikes the the top surface of the wing, and then moves off in a straight line, there would be lower pressure on the top surface of the wing. For this not to happen, the wind must flow around the surface of the wing.
You can also think about it as if the air above the wing has higher pressure than the air just on its surface due to the motion of the air, and so it gets pushed down onto the wing.