Look, I know that I'm a bit late to the dance on this, but if you're best friend, Harry, has a brother, Larry, and five days from now, he's gonna marry....
     Why the hell would he ask YOU be the best man?
     Does Larry have some strange, adversarial relationship with Harry and want to spite him by inviting you to be his most trusted confidant? Does Harry suffer from some degenerative disease that does not allow him to travel distances? Does Harry hate Larry's fiance and you are the next best thing since he refuses to attend? Does Harry not believe in the institution of marriage? Perhaps he's incarcerated on on board the international Space Station. Perhaps Harry is Debbie Harry, lead singer of Blondie, and they are  currently on an international tour. Any way you slice it, you're doing someone your best friend's family a disservice by doing this. It's really best in situations like this to stay out of other people family affairs. Still, why would Larry ask something like this of you. Doesn't he know that this is just going to cause unneeded complications if your life? Perhaps that had been part of Larry's insidious plan all along. 
But why? 
What is the deal with this? 
WHAT? I must know!

This world is just to frustrating. Sometimes my first inclination is to become a monk and leave the situation.  

That'd show 'em. That'd show 'em all.