Nothing hurts you more than seeing him again.
You've lost your share of companions, but that doesn't mean his loss made any less of a hole in your heart.
You remember when his brother confronted you in the yard. It had been after a particularly exciting adventure- he was already asleep in his room, and you were sneaking out the back before anyone noticed you. Unfortunately, Mycroft did. You remember him telling you that your little adventures wasn't helping anything- if anything, their mummy was more concerned about the boy than she had before you were ever in the picture. Apparently Sherlock was always going on about space and time, aliens, and almost dying. Apparently now Sherlock wanted to be a Pirate, all because of that space ship you landed on by accident. Apparently Sherlock was rude- telling people they weren't very clever, or that they weren't real Doctors. Apparently imagination was good for little boys but Sherlock had too much, and it was all your fault.
You remember leaving that night, not even saying goodbye (not like you had with Amy. This time, there would be no coming back- and despite the fact you told yourself that, you couldn't help but wish that wasn't true.). You had looked over your shoulder at the big house, Mycroft looking at you disapprovingly with his arms crossed. You had thought of Sherlock that night, how he was blissfully sleeping in his room, not knowing there wasn't going to be a next adventure waiting for him.
You remember seeing him as he grew older. You remember watching him mingle with other children, only to drive them away within moments of talking to them, and it broke your heart to see his intelligence wasted on unappreciative humans. You were painfully aware as he began to experiment with drugs- even to overdose- and you wondered if it was really for the best that you left. You left him flowers in the hospital (though you knew he wouldn't appreciate or understand the sentiment), but you never stepped in to help him. You never talked to him. And you hated yourself for it.
You will never forget the brilliant little boy, who was much too old to be so young (much like yourself, you think sometimes). The boy who confused you as to whether you thought of him as a brother or a son. You will always remember how he grasped concepts such as the TARDIS with relative ease, how he had the ability to read people as if they were picture books.
You will never forget Sherlock Holmes, which is why it hurts so much to see him again, running in the street. He has a companion of his own now, a Doctor by the name of John Watson. You couldn't be happier that he didn't feel alone anymore- although, when you really think about it, he never was. He had Mycroft from the very beginning, however aloof the older brother might have acted. He had Lestrade(who looks awfully familiar) and Miss Molly Hooper- who you think might have made a swell companion. He had you.
You see him again, and, although you know you shouldn't be disappointed if he didn't recognize you (you have a different face, after all), you can't help but feel crushed when all you receive from him is a cold glare, and an even colder shoulder.