Friends are those who have given me enough data about their own lives to be left afterward trusting me.
Sounds odd when it's put like that, but it denotes that we've (me + friend) had enough time together for me to learn a lot about them, that I found the things they had to say interesting enough for me to stick around instead of find someone cooler, it shows a relationship where thoughts are put out in the open in discussions instead of veiled out of insecurity, it shows that, from the context they made from talking about themselves, that there is an unspoken understanding as to why they'd say something (subtext~), and it shows they can stand my company long enough for all of that to be possible.
Usually they learn about me too during the process, and if that doesn't scare them off, it usually turns into a pretty chill co-existance where we do stuff that we both enjoy.
Decision? Not usually. I just ask the right questions, and let them keep talking until I've heard enough to make a personality profile. By the time they realize everything they've said, they usually see me as a good listener, a source to vent at, and someone who has interesting input from time to time about their lives once I pull together past and present references, whether to invoke irony or establish a theme.
I think it's closer to me just looking into it more than is necissary. In simpler terms, a friend is someone who trusts me, even though it may be a question to them as to if I trust them back (I have some trust issues).
It's closer to me gathering a band of misfits who see my not buckling from disorder as inspirational, if not simply appreciate how random my tangents can be, along with the occational normal person who appreciate the "stoicism". Being mostly unaffected by drama has them see me as someone they can flee to for calm discussion, and the blunt honesty tends be a nice break from "all the bullshit".
I'm not really nice, I'm just polite and blunt interchangably or at once. I have frienemies as well as former ones who outright became "enemies", for lack of a less dramatic word. The one thing in common between them all is that they are warped individuals who find my very being distracting toward the world they built around themselves perceptually.
Come to think of it, a lot of those who don't stick around seem to do so out of fear that my views might change them. Those who stick around... seem to change to become something closer to who I am. It's kind of creepy.
Edit:
"and will sort of come and go whenever you get involved in something else"
Yeah... I'm pretty guilty of this.
There is a guy I hang out with occasionally online. Even though I ignore him when I don't feel like spending time with him, which is most of the time, he still insists on being friends with me. I'm certain it's because we're similar in some ways - quiet, reserved and unemotional (unless someone pisses us off) - that he and I get a long so well.
It's the ideal 'friendship'.
He's content to be around when I want him to be and accepting of the times when I don't.