Spitfire - Captain, lead flyer and idiot-wrangler of the Wonderbolts - has a reputation. Whether she's whipping her wingmates into shape, keeping her trainees in line or sometimes-intentionally scaring little kids, it's hard for many to imagine her not either shouting or being ready to shout at a moment's notice.
That's not to say ponies disapprove. Even ignoring the many times the job or the crowd demand that sort of display, it's not unreasonable to assume she must be under a lot of stress, so few will begrudge her a bit of a raised voice. Still, regardless of the reasons, this does mean that, to most, she is a very scary mare.
It is therefore good, on multiple levels, that no one knows about her secret meetings with one of the most easily scared ponies in Equestria or about what truly happens within her cottage.
An entry in Bicyclette's Crackship Contest that... I'm not at all satisfied with and basically shamed myself into writing. Here's hoping I didn't screw it up too badly.