Disclaimer: I do not own NCIS, nor am I making any profit off of this.
~oOoOoOo~
Gibbs thought about Shannon and Kelly more often than one might expect him to, even if one was really close to him (which wasn't a lot people; he generally kept most people at a distance nowadays). Not even Ducky or Jenny could have known. There were days, however, when he could be making a new boat, drinking some Bourbon or otherwise working on a high-profile case that he wasn't reminded of the stupidest mistake he'd ever made.
That mistake had been in leaving them alone when he'd gone off to fight in the Middle East. His heart was almost ripped out when he saw the sadness in Shannon's eyes and hearing Kelly cry as she begged him to stay. It had been easier to leave the first time he went off to war - the only thing he was leaving was a small town he despised and a girl he dreamed of seeing again, if he lived to actually see her again.
And then he sat in the back of a truck, watching as he left his entire world behind, and for what? To fight for something he had believed in when he didn't have anything to lose. At his young age, a carelessness and a wish to prove himself had pushed him into joining the Marines years before, but now he left simply because they wanted him to.
He could remember them sometimes, if the house was quiet enough and he had had enough Bourbon for him to hear Kelly's song-like voice, calling out to him and asking him to play with her, and Shannon's laughter as she watched her daughter hang on his arm all day. He could remember when they had ridden horses on the beach, and watched television in the living room, and when he sang to Kelly.
He could also remember when Franks told him the bad news after he woke up from his coma; when he left to "take a leak" and he read the case file on where NIS believed his wife and daughter's murderers to be located. And he could remember killing him just like it was yesterday.
It was memories like these that kept other women away, and why he kept them away. After Shannon and Kelly were murdered an overwhelming pain and endless depression overtook him, turning him into a quieter, stiffer person than he had been before. Even when he married again, and he married three more times after that, he never told them about Shannon or Kelly, about how he had gone down to Mexico with his sniper rifle and shot the bastard, about anything. His lack of conversation and soul-searching drove them into believing that he didn't care. But he did care - he just still cared about Shannon and Kelly.
Often he wondered whether it was his stiffness that had driven Jenny away during their time in France, or if she hadn't planned on falling for him in the first place. He decided it could be both (and thus the reason for his rule: never date a co-worker). As a goal-oriented and strongheaded woman, Jenny rarely backed down when she was focused on something she wanted, and usually hurt others in the process. Gibbs didn't actually believe that things would have worked out between them, but he thought things could have gone better.
Sometimes, though, these painful memories had a useful purpose instead of attempting to drive him further into life-long depression. He found he could relate better to victims or even suspects on their various issues with parents, spouses and children. And his understanding helped him to wheedle out the liars from the honest people in the interrogation room. Gibbs could end up being both a bastard and a saint, depending on the situation.
If anyone was ever brave enough to ask him, though, in all seriousness, if he thought that he would ever be able to let go of Shannon and Kelly, and move on with his life, he would say no. No, he wouldn't.