What draws you to a person as a friend?

I was thinking of some friends of mine and how much I admire what they know and the way their minds work. It made me think about friendship and what attracts you to someone initially as a friend. Is it the ways they are like you? What you can learn from them? A shared sense of humor perhaps? Or personal qualities llike loyalty, introspection or compassion? Does what drew you to the person initially sustain the friendship or do other factors change it as it developes?