Jack’s spent a lot of time in uniform, first with the Time Agency then fighting for one army or another on various worlds. He’s seen action in most major wars on earth over the past century and a half; you’d think by now he’d be tired of everything to do with warfare.
Yet he’s most comfortable dressed in the style of a WWII aviator. He doesn’t wear the full uniform, although he owns everything right down to the cap, but he always dresses the same even if he can’t explain his affinity for the era.
Besides, everyone loves the coat!