Neither of them ever thought that they would actually cross the line. Yes, each of them had imagined it plenty of times and thought about it off and on for a little over seven years, but still they had never crossed that line. Both of them found the other extremely attractive but there was always something that held them back.
At first there was so much chaos in both their lives and the fact that they were supposed to be enemies. She was there to protect her brother and he was trying to avenge the death of a woman who he cared for as a sister.
Then they became work associates; they couldn't call each other partners yet. They danced around each other and flirted for months wondering who would give in first. But they waited too long because the next thing they knew, Gibbs had blown up and forgot them all. He left and went to Mexico. That summer was the opportune moment to finally do it, what with Gibbs no longer in the equation. Ironically that's the reason they didn't do it. Without Gibbs they needed each other as something to stabilize themselves and if they were romantically involved they wouldn't have had that.
Then she got in trouble and called Gibbs, not him. For some reason that hurt. Gibbs came back, but then the Jeanne situation unfolded and it was her who felt hurt. But when the Jeanne situation diffused and when he could have sought her he didn't. And she never offered herself as a shoulder to cry on.
He suffered alone until he was over Jeanne. Then the two went back to the way it was before, flirting and dancing around each other. Close, but never close enough. Then Jenny died and he blamed himself. He was shipped off to sea and she was forced to return to what she once called home.
They could have slept together as a goodbye, both of them thought of it. But when they went to say their goodbyes they only hugged. Albeit it was a long hug that promised more than just friendship, but they never did more than that.
Finally they were reunited, but this time things were different. This time she was in a relationship and he was the one wanting her. She knew that all he would've had to do to be with her was to ask, but he never did, so she stayed involved with Michael. Then Tony got jealous and protective and Ziva got angry and defensive and somehow during all of this Michael died and Ziva was heartbroken, but not nearly as much as she ought to be.
She left and went to die in the desert because that's what she thought she deserved. But he saved her and brought her back and though he wanted it now more than ever, he knew that she was in no state to start something romantic, with him least of all.
He gave her plenty of time to recover, a whole year. He told himself that he would wait until she became a U.S. citizen and then they would try something, but Gibbs sent him away while she became an American and he still wonders if Gibbs knew what would have happened had he stayed.
He realizes too late that he waited too long. Because when the summer is over she's already found a new man. And he's okay with that because he knows he probably wouldn't have crossed the line anyway. So he goes out with a few women while she goes steady with a man whose name she won't divulge. While they each do this, their own lives are crazy, absolutely crazy. She saves him from a bomb going off. His father comes and is the main suspect in a murder. Her father comes from across the sea, not because he wants to reconcile, but for business. Each of them tries to move on and start over with their fathers. Then they each get their minds picked and prodded by a shrink and it brings into the light their dysfunctional relationship and each of them are caught off guard by the returning feelings towards each other.
For a little while their 'relationship' cools down and they become more like sister and brother then almost lovers. He has E.J. who despite everything on the contrary, Ziva doesn't trust. She had Ray who Tony is extremely watchful because the last thing the team needs is another Michael incident. But somewhere deep down Tony knows that something bad will come from C.I. Ray.
But then it all starts coming crumbling down after Mike Franks dies. Everything has been pulled out from under them and they both feel too much pain for their own good. She cries and cries and for the first time that either of them can remember, she is showing him her vulnerable side with not a single defense. He holds her and tells her everything's going to be fine. He doesn't know for sure, but he's going to make damn sure she'll be okay. Then she gets taken and he knows that she's in danger. He goes crazy, and in the hours before he knows she's okay, he reflects. And he thinks that maybe their song has come to an end. That maybe they'll never dance together again, and he can't bear the thought.
So when it's all over. When the P2P killer is caught. When E.J. and Ray are gone. After Franks' funeral. Before Tony is approached by the new Secretary of the Navy. He takes Ziva home with him.
That is where they are now. This is when they cross the line they've been at the edge of since they met. They sleep together and it's amazing and it's also a bit strange. After the years of foreplay and resisting, they finally do it. And who knows what'll happen to them. They might only be together this once. They might try this for a while and then end it. They might break each other's hearts and move away to escape the pain. Or maybe, just maybe, they'll live happily ever after. Maybe that's why they've waited until now to do anything. But one thing's for sure and that's that while they're together, while they're moderately safe, they will enjoy each other and they will not regret because they are in love, even though they're both too stubborn to admit it.
Gibbs has had much time to contemplate the relationship that goes on between Tony and Ziva. It's been an experience watching them, wondering how long it would take to reach this inevitable point and Gibbs truly believes that it was inevitable. It's also a bit strange for him because although Tony looks at Ziva the way Gibbs looked at Jenny, the way both reacted is completely different. Jenny and him slept together almost immediately and they never thought to fight it. But she had no idea how much she hurt him when she left and that's why the rule number 12 even exists. He doesn't want his team torn apart the way his heart was. But though he knows what's happening between Tony and Ziva, that they're crossing the line, he doesn't want to stop them anymore. Maybe, he thinks, that because they waited so long, it just might work.