What is the one thing every single friendship has?

Is there one thing every friendship has? Let's have a look and see by example if we can find something.
Alcohol
I used to consider this an element of common ground. As I've grown older, more and more of my friends have given up drinking. Some have developed problem relationships with alcohol and have had to give it up.
Hobbies
My hobbies have spanned a wide range of interests over the years. Kite flying. Radio control cars. Ham radio. Mountain biking. Road biking. Invention. Wood working. Wine. Food. Art. Science. The list goes on and on and there is not a single person I know who has shared them all. I doubt there are many on the planet who have shared them all. Hobbies are sometimes an element, but not always.
Likeness
We all tend to hang with people we feel are "like" us. They have a familiarity. We often have friends in common. I don't know about you, but there are a whole lot of square pegs and weirdos I am REALLY fond of. Likeness may have its appeal, but opposites still attract.
Brains
I really like smart people. It's the reason I practice a vocation that requires a license that is really difficult to obtain. It's why I run a Meetup group for inventors and why most of you are my friends. Still, I'm also pretty fond of some people who won't be winning any spelling bees or math contests.
Contact
Define contact. If you mean face to face communication, phone calls or Skype, this one goes out the window. I have friends all over the world that I only know from online communication. Message boards, IMs and Tweets make up a good 20% of the contact I have with my friends from day to day.
In the end the only thing I can think of is an inexplicable desire to be someone's friend. For one guy it's because he rides a titanium bike. For another, it's because he built a speaker that has dancing flames coming out the top of it. For one gal, it's because she's fearless. For another because she's...she's...oh, hell I can't even describe it! Maybe it's just because she loves me and trusts me. You know who you are.
Maybe there isn't one single thing common to every friendship I have. Oh, wait. There is.
Me.