John had realized a long time ago everything that Sherlock had done for him. He had gotten him a flat, a job, and off of that stupid cane, to name just a few of the obvious things. But there were times that he would wonder if he had done anything for Sherlock. He would go over it in his head again and again, but the only thing he could ever think of was "I get him milk" and other petty household things. He didn't think that he could've affected the consulting detective's life in any important way.
It took Sherlock a tad longer to see that John had affected his life. Oh, he had noticed that he now had someone that wasn't completely annoyed at him all the time, but as time went on, he saw that it went deeper than that. He had stopped using the nicotine patches, and started to drink coffee. He had stopped smoking altogether, and he had also started eating more. All because of John. All because of this one man that had somehow managed to break down his wall of science, and deductions, and logic.
They had a mutual understanding that they had reached the point where they couldn't live without the other. Though not romantic at first (John had been quite adamant about his heterosexuality), when simply working together on cases wasn't enough anymore, they took it to the next level, and were surprised at how natural and right it felt. If asked, neither would admit to all of the things the other does for them, but it's there, in their eyes, when they look at each other.
Their silent support.
Their understood love.
Their unspoken "thank you".