As a rule, I generally don't give much thought towards quantifying my varying degrees of closeness to my friends, but for discussion's sake, it did make for an interesting topic: what does make a close friend close?
Is it the person with whom you spend the most time with? The person to whom you share the most with? The person you talk to the most? The person you have the most in common with? The person with whom you have been friends for the longest?
There are endless reasons to be close to someone, and indeed all the above reasons are reasons to be grateful for. For me, there are two reasons that I am close to the people I am close to.
The first is honesty. Among those I count as my closest friends are those who are willing to open up to me. They have shared things with me I know that one does not immediately readily share with the world, and their trust and vulnerability induces reciprocality in me. I have great difficulty in sharing and opening up to people, and as I have grown older I have only made more mistakes in whom I elect to share my vulnerabilities with. This negative feedback means that I tend only to open up to people I feel have opened up to me.
However, more than just sharing their personal lives, such friends are treasured for the fact that they are unafraid to share their personal opinions, even when said opinions may be less than savory. When someone is willing to insult or rebuke you in the interest of being completely honest, it is for me one of the signs of a true friend. Caveat: insults alone do not imply friendship though - quite often they imply rather the opposite.
The second is gratitude. As my dear blogstalkers are well aware of by now, I love a good cliche, especially when it is true. And my dears, a friend in need really is a friend indeed. This works both ways. Both those who have helped me when I am weak and those who have allowed me to see and aid them in their darkest moments, are friends who I treasure deeply.