Nick's right. For as long as you keep thinking about your friend and remembering the good times, a part of him lives on in you.
Like most of us on this forum, I suppose (given the typical age of its membership), I too have lost people I was close to over the years. It's horrible when it happens, but you're quite right, it can be frightening just how fast the time flies by. The immediacy of the pain and loss thankfully diminishes, but the sorrow that they have departed never really goes away, and can creep up on you and surprise you into sadness again quite unexpectedly. However, remembering the good things about those people and how much they meant to us can bring some comfort. I find myself remembering the funny things they said most of all. Just this afternoon I thought of an hilarious insult thrown my way in jest by someone I still miss terribly, and it raised a smile on my face thinking about the gleeful way they deliberately made me look stupid