It is a uniform principle that anything in this universe would tend to remain in the lowest potential energy possible.

When we rise the bucket above to a certain level, then it had a gain in its potential energy.

Since, it would want to remain in the lowest potential energy possible, so the gain in potential energy is converted into the kinetic energy and hence it moves down. So, at every moment, the bucket wants to minimise its potential energy and move down, but since we want to maintain a certain level, therefore a certain force needs to be applied such that

F=mg

Remember, the work done here is zero, but a force is needed to keep that body at thet level.

We are not able to hold the bucket at the constant level as the potential energy would change into the kinetic energy.

Please Approve!!

this phenomenon is practically not possible.as we r applying force opposite to that of gravitational force.thus action reaction pair forms.till the reaction pair(force applied by u)is working the net work done will be zero.as u can't apply thatb force for ever so this is practically not possible.