why the electrons flow from the ground to the sphere when we trying to charge a metal sphere negatively?

In electrical circuits, ground refers to infinite source or sink of charge. We know that sphere at first is neutral with equal number of positive and negative charges in it.When we want to charge a sphere , say positively,we bring a negatively charged rod near to it. While doing this, the excess electrons in the sphere flow to the ground which behaves as an infinite sink of charges. Here the sphere becomes positively charged by induction. 

In this similar way , we can also negatively charge a sphere. If we bring a positively charged object  near the sphere which was neutral in the beginning, then , due to induction, negative charge will develop on the sphere's surface. This excess of negative charge is compensated by flow of negatively charged electrons from the ground which now behaves like an infinite source of charge.