A car is a complicated machine, yet its tremendous power is derived from a single spark that sets the engine in motion. Nevertheless, it is not good for the motor if the starter's sparking action is used too often.

This is analogous to a Jew's spiritual life. His purpose is to serve G-d, using the strength that the Almighty vested in his soul. Though it is true that from time to time, a person is "ignited" with an external spark that inspires him in his relationship to G-d, he must not rely on this stimulation as an impetus for divine service. Instead, it is better for a person to internalize the original burst of energy, serving the Creator with the ongoing potential and ability with which he has been imbued.

Igros Kodesh of the Rebbe, Vol. 7, p. 46