John,
My father once tried to explain to me what love truly is. I of course couldn't have cared less at the time, but for some reason, the words he said have stayed with me.
'Sherlock', he told me, 'to truly love someone is to want their happiness even more than your own.'
I never understood. All my life, I never understood. Not even when I met you and we moved into Baker Street, or when you shot the cab driver on our first case together. Not when I surprised even myself by being so outright worries about you the night Moriarty strapped a bomb to your chest. Not when I spent all my time trying to understand you when I normally would have gotten bored and given up. I did not understand, even when I left for two years, letting you believe I was dead. Not even when I heard you speak at what you thought was my grave. I never understood.
I do now. I know how selfish I have been. And now here you are on your wedding day, and here I am writing a letter you will never see. But you must know, John, that when I say that Mary and I are the two people who love you most in this world, it is true. And you must know that your happiness has come to be more important to me than anything ever has.
Sherlock