NCIS Fans Think This Is The Origin Of Gibbs' Rule 12

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.