I've been thinking about this on and off, but until now I don't have one answer that encompasses everything.
However, here's my best attempt in figuring out which qualities attract me to my friends and keep us together:
1) Similarities in our interests, particularly in fandoms and in creative pursuits. My blogroll here alone is full of fans of anime, manga, cosplay, Japanese fashion, photography, graphic design, and food, among many other things; same goes for my blogs in other "places," like Livejournal.
2) Varied life experiences, often those which are so vast and expansive and different from my own. Because they've allowed themselves to experience all that life has to offer, they are particularly open-minded. I will admit that my experiences are very limited due to my sheltered circumstances and my tendency to close myself from the rest of the world, and now that I think about it that's why I find myself drawn to, and highly appreciative of, people whose opinions, while contrary, are very well-formed, and people in whom I can live vicariously (did I spell this right?). In particular, I'm with people who are well-traveled, people who constantly travel, and people who love to travel.
Case in point: a co-worker read the title of this post as he was passing by my workstation, and he succinctly told me that there was no such thing as "writer's block". When I asked him why, he crossed his arms behind his head so he could lean on his hands and answered with a slight smirk, "Because if someone puts a gun to your head and demands you to write something, you would, no matter how short it is!" Hmmm, makes perfect sense actually. Outside of work, this man is a fiction writer whose work I've read and admired, who once lived, studied and even worked abroad before returning here to work in my present company (those of you in my circle who know exactly who I'm talking about, shhh ;P).
3) Temperaments which are much less volatile than my own, for very obvious reasons.
4) Smiling ways and a great sense of humour. I love surrounding myself with happy things and happy people
5) High degrees of tolerance for my insanity, my inconsistencies, my rage (especially in times when that long-withheld anger is set off by something so slight), and for those days when I can't even bear being around myself.
These are my answers, off the top of my head.
Everytime I think about my friends, I keep reminding myself that I'm lucky I've got friends at all.
So, for those friends of mine who read this because they felt like reading it, I'd like to take this opportunity to thank you for being my friends, even if it's just online. v^_^v