why engines are mounted downside of the wing?

Is it necessary to mount the engine beneath the wing?YES.
But it may depend on the aircraft (small/large), because small airlines  have engine at the back of the aircraft.
Usually commercial airliners mount the engine beneath the wings;
so what is the reason?

Engines mounted beneath the wing

PROS:

• Easier access to the engines parts.
• Maintenance stuff like(changing oil,cleaning the dust particles and etc) becomes handy for the worker.
• Engines can be separated easily from aircraft when something damage occur to it.
• Supply of fuel from the wing system doesn't make it complex(i.e piping of fuel system).

CONS:

• High noise disturbance for passengers.
• Ground dirt can easily enter into engine.
• Requires larger rudder for yawing.

  

  Engines mounted beneath the wing.
  Boeing 737  aircraft.

1.     Engines mounted at back

Pros:

• Can reverse the aircraft easily on the ground.
• Less likely to take debris object from the ground.

Cons:

• Harder to access for maintenance,
•  Requires more structural support and longer fuel lines,
•  More dangerous in case of fire.

  

  Engines mounted at the back of aircraft.
  Md (Mc Donnell-Douglas)80 aircraft.