The sun is able to radiate energy (or, in the words of the first chapter of Genesis, "to give light to the earth"). If the sun had the same temperature and energy but did not radiate its heat, it would be of no value to our planet.
So it is with the Jew, whose main function is to shed spiritual light, benefiting his fellow man through the mitzvah of Ahavas Yisrael (love of fellow Jews). Without the capacity to reach out to others, a Jew would function as a "black hole" instead of the luminous star that he was created to be.
As related by Herman Branover in Return, p. 173