I think Mac sees Stella as a fellow "warrior". Someone who could be very dangerous given the right circumstances -- and dangerous in a determined, deliberate way. Danny is hotheaded and Flack is no one to mess with, but I think Mac views Stella in the same way he would have viewed a fellow Marine in his unit. It's a respect that I don't see Mac giving anyone else.
The show allows Stella to be intense, a characteristic that is rarely seen in women characters on TV. Women are allowed to be emotional or intuitive or tough. They can be dangerous, but usually only when they are very emotional. They are rarely allowed to be intense in the "don't mess with me, I'll take you out" way that Stella has. When Stella shot Frankie she wasn't acting out of any hysterical, over-wrought way. As scared as she was, she was still firmly in control of her actions. Stella's character is as equally strong and intense as Mac's. That, IMO, makes them the most interesting character dynamic on the show.