It was unquestionable, something taken for granted by both men. They could always be depended on to have each other's six. It was an unspoken promise between them, as so many things were. It what was allowed them to go into uncertain and dangerous situations without ever checking that the other person was by their side. They would follow each other into hell without a question. It was part of what made up their unique dynamic. Someone on the outside looking in would probably never notice just how deep their mutual trust went. And it was deep, bone deep.
Neither person had ever been good at communicating their wants, thoughts, or desires in a relationship. Perhaps why they were both so committed to this relationship. They didn't need to communicate; they knew beyond a doubt what the other person was feeling. Even at work in the middle of a case an unconscious connection hummed between them. Tony always knew when Gibbs needed some coffee or a joke to lighten the tension that seemed to settle during tough cases. Gibbs always knew when Tony was getting too deeply affected by the case or an undercover job and needed something to bring him out of it.
They were always willing to do whatever it took to bring happiness to the other man. Another thing far different from their previous relationships, nothing was held back. They gave all that they were, had been, and ever would be to the other, instinctively knowing that doing so would mean that they would never separate, together even in death.
They kept no secrets from each other, and so when Tony needed a reminder of his place in Gibbs' life he got it. Kneeling silently at his master's feet, forbidden to talk, he was told to think on his position. Thinking back to what had brought them to this point, he could see now that it had been inevitable. So comfortable with each other from the start, this dynamic had fallen into place almost subtly, until one day it had just clicked, and Tony had first found himself in this situation.
He had never wished to be anywhere else, and in fact would stay there contently forever if he could. It was such a small thing, but it was an immense comfort, a surety in his life that he depended on for balance. It was how he defined himself; he was Gibbs', nothing more, nothing less. It was all that mattered; as long as he had that he didn't need anything else. Not something he had ever thought he would want, it made all the difference in the world. He found himself more self assured and content than he had ever been in his life, and all because Gibbs had made such a place for him.
He was comfortable in his skin, and his life, with no uncertainty where he would be years done the road. Tony didn't even need to think, he knew where he would be. The same place he was now, by his master's side, facing whatever would come as they should be, together.