A hypodermic needle draws blood for the purpose of diagnosis and treatment. However, it is not the needle which draws the blood from the veins. Rather it is the vacuum in the syringe.

The lesson of the value of a vacuum can be extremely relevant to a person who considers himself “empty,” \endash unworthy \endash to be successful in his G‑dly service. An empty vessel can draw in with greater intensity than one which is full. So too can the person who is aware of his own inadequacy be more strongly motivated to study and to do positive things.

Similarly, when one finds oneself in a situation where an absence or loss is deeply felt, one need not be despondent. We can rather use the emptiness itself as an impetus for even greater achievement.

Adapted from a message of the Rebbe, Simchas Torah5738,
quoted in Toward a Meaningful Life,p. 265