Question

Where do objects becomes weightless and why?

Answer 1

Object become weightless in space and in water. gravity is what holds an object down, without gravity object would float around.

Answer 2

           Weight = (mass) x (local acceleration of gravity)

The local acceleration of gravity depends on the other masses
around you and your distance from them.  So an object can only be
truly weightless when it's infinitely far away from any other mass.

But an object appears to be weightless when it's in free fall, that is,
when the only force acting on it is the force of gravity.  In that situation,
the sensation of 'weight' is impossible ... you can never feel that something
is pressing against you, because everything else is also in free fall. 
So a scale on the floor can't exert force against your feet, the mattress
on your bed can't exert force against your back, and your heart, liver,
stomach and spleen can't exert force against your other guts. 
Nothing seems to have weight, because everything is falling with
the same acceleration.