I rewatched "Wasted" from CSI:NY S2 today, and I'm more convinced than ever that Flack has a bugaboo/soft spot for the sick or physically weak and their loved ones. He had a very strong reaction to Paul Ridgemont's revelation that he was terminally ill, and he curiously displayed no snark whatsoever in that episode. Additionally, when Lynette Ridgemont was spinning her tale of woe to Flack and Stella, Flack had an unusually empathetic expression, as if he knew from experience how the story went. Now I know Flack is a fundamentally decent guy, but it seemed especially subdued given his penchant for swagger.
So, is it possible that he has a the soft spot on account of his parents? Maybe he watched his mother die of a long and painful illness, like cancer, or maybe his father was forced into retirement by MS, ALS, or Parkinson's. If so, and if Flack has borne witness to the slow decline of his NYPD-legend father, it could explain his terse response to Gavin Moran in "The Fall" when Moran asked after Don Sr. Maybe he doesn't want to tell anyone that his father is falling apart, or maybe he's afraid that he'll wind up in the same boat. Or maybe there was a sibling he lost to leukemia.
Anyone? Anyone? Bueller?