This lesson came from Sarah MacLean, although she was quoting the great Linda Howard:
If your hero is a firefighter, your heroine better be an arsonist.
I’ve heard this said in other ways before, but the way Howard put it is a hundred kinds of brilliant. The best romantic couples are the ones who start off so different, you just can’t possibly imagine them ever getting together. But at the same time, they are so in dire need of each other, you also can’t imagine them ever being apart.