You're my best friend
When I was in elementary school one of my friends asked me "Are we best friends?" I was shocked at the question because I had never thought of labeling any of my friends. I assumed because we hung out all the time that we were GREAT friends. Adding a title to our friendship was scary because this would be a huge commitment. From now on we would tell people this, I would be in his wedding, he could always sleep over, we help each other talk to girls and we would give each other answers on test questions. All this ran through my head. He noticed the pause, and was hurt that my reply was so delayed.
Recently I was married and asked a long time high school friend to be my best man. He was PUMPED! We didn't talk much in college, which was ok, but after the wedding we talk often. I think that by telling him how special he is to me opened up our relationship even more. I was willing to be vulnerable, admit his importance in my life, and strengthened our friendship.
Don't ask someone, just tell someone they're special.