NCIS Fans Think This Is The Origin Of Gibbs' Rule 12
Now that he's been leading the team on "NCIS" for nearly two decades, fans of the show have come to know quite a bit about Leroy Jethro Gibbs (Mark Harmon). He's a famously terse character who never says more than he has to, and yet, his leadership style clearly inspires a deep sense of loyalty in the team he leads. Over the course of the series, we've also learned more about Gibbs' past, including who his father was and where he grew up.
Although Gibbs is not a talkative guy, he's famous for the 51 rules (via CBS) that he has for himself and his team. The rules cover many different aspects of the job and cover everything from never letting suspects sit together to always being as specific as possible when you lie.
These rules all serve a specific function, and Gibbs presumably has a reason for each of the boundaries that he sets. Those reasons may be logical, or they may have come from hard-won experience from his own past as an NCIS agent or his time in the Marines. Some fans have speculated that one rule in particular, Rule 12, was created because of one specific experience from Gibbs' past. The rule states that you should never date a co-worker, and it was first introduced in Season 1, which debuted in 2003.
Gibbs may have created Rule 12 because of his own heartbreak
Gibbs never says for sure why Rule 12 exists, but some fans on Reddit suspect that it was created in the wake of his experience with Jenny Shepard (Lauren Holly), a former NCIS director that he had an affair with. The two of them slept together in 1999, four years before the first episode of the show.
Their history together, and the fact that Jenny breaks off their relationship to further her career and because he can't commit to their future as a couple, may ultimately explain why Gibbs now believes it's important that agents never date one another. Although there are times when his team violates this rule, Gibbs' own heartbreak may justify why it was created in the first place. He knows that it can break a unit apart, and lead members of the team to be resentful of one another.
It's hard to trace all of Gibbs' 51 rules back to a concrete experience from his own past. In the case of Rule 12, though, the reason for its existence seems very clear.