I think that part of the reason we are having this debate and searching for reasons whether sadness is, or is not, appropriate, is that we are conflating "once in a lifetime" with "uncommon". Anyone born prior to 1938, like my parents, have witnessed six calendar year slams in singles (1938, 1953, 1962, 1969, 1970 & 1988). Certainly not "once in a lifetime". Now, Halley's Comet, which most recently appeared in 1986 and will next appear in July 2061 is a once in a lifetime event for most people. Moreover, anyone born today will have +/- 160 chances (80 for each gender) to witness a calendar year singles grand slam but only one shot at witnessing Halley's Comet. So those who are sad at Djokovic's missed opportunity can take comfort in knowing that there will be two more chances to witness a CYGS next year!