Paraphrase this epigram from Emerson's "Nature": "To a man laboring under calamity, the heat of his own fire hath sadness in it."

This saying makes me think of the fire fighters, police officers, and other rescue workers who searched the debris of the Twin Towers for survivors. The collapse of those buildings can certainly be called a calamity (really, that's an understatement). The people looking for any sign of life were heated, or extremely eager, to find someone alive. Yet, at the same time they were extremely sad because they knew that many people were dead and that it would be unlikely for them to find survivors.

You might paraphrase this statement as: To someone working under severe circumstances, the desire to finish the job comes with sadness.