Why are the leading edges of wings not always made as 'sharp' as possible?
A blunter (less sharp) leading edge allows the wing to operate effectively for a wider range of angles of attack (AoA). The angle of attack is the angle between the approaching airflow and the chord line (as illustrated below)



As you can see, the air has to curve around the leading edge on the upper side of the airfoil. This curvature accelerates the flow and thus creates an area of reduced pressure and thus contributes to the lift of the airfoil.

A sharper leading edge will cause a more intense curvature of the flow and therefore reduce the local pressure even further.
A higher angle of attack will also cause a more intense curvature of the flow with the same effects.

Here's the problem: In order to reach pressure equality between the upper and lower side at the trailing edge of the wing, the pressure will have to increase again as you move further towards the trailing edge.
The boundary layer of the flow doesn't like this at all. Strong pressure gradients will cause the boundary layer detach (generally, the boundary layer is already turbulent for commercial aircraft). A detached boundary layer (stall) causes a drastic increase in drag and decrease in lift and is therefore highly undesirable. 

To increase the range of angles of attack at wich the boundary layer stays attached and to allow for a more gradual transition between normal flow and stall, the designers favour a quite blunt leading edge for most commercial and general aviation aircraft. This will increase the drag at zero AoA, but won't force the air to follow a high curvature at positive AoAs.

Supersonic aircraft such as the Concorde and fighter aircraft have to deal with a new type of drag called wave drag. Generally wave drag can be reduced by having sharp leading edges. 
Most boats also have to fight against some kind of wave drag (although the fluid dynamics are very different) and therefore also have sharp hulls.

This is a broad generalisation and there are a lot more things to consider in reality. If you want I can go into more detail.