A calamity is:
- An event that brings terrible loss, lasting distress, or severe affliction; a disaster: A hurricane would be a calamity for this low-lying coastal region.
- Dire distress resulting from loss or tragedy. (www.dictionary.com)
If I had to paraphrase this epigram, it might read something like this:
Humankind faces many adversities in their lives; these include losses of loved ones, financial difficulties, etc. Despite having to face these, humankind still has the desire to carry on and power through these difficulties; however, doing so will be with a sense of sadness.
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.