There were things John Watson did for Sherlock Holmes that he would do for any of his few but fond friends. Traveling halfway across London by request of text message, playing messenger to family members, and covering the occasional cab fare were such things upon which any good friend of John's could rely.
Certain things John would only do for Sherlock. Some were obvious, like killing a man and staying up for two days draught to track down a serial killer, which were the sort of thing any good friend of Sherlock's would do, if the man had friends besides John. Some things Sherlock asked John to do and only John would do them.
Once, John put Sherlock's shoes on for him because he was composing a text message. Later that week, John held the CTRL key on Sherlock's laptop because he was splayed in such a way that only one hand could reach the keyboard and he couldn't be bothered to move his other arm to copy a URL.
Of all the things, big and small, that John only did for Sherlock, the only one he could never complain of was loving him so completely that he couldn't even be angry when Sherlock forgot he was texting to revel in his shoes going on his feet without his participation.