When I first meet someone, I honestly don't think about the long-term friendship. I'm more of a "what do you offer me here & now?" Can they make me laugh? Do they have a hobby similar to mine? What is it that we have in common that we can enjoy together? I look for something, anything that can connect me with the other person. I don't mind if it means that we're only light friends, the hello-goodbye-how-are-you-oh let's eat together today~ kind of friends. And if, over time, I can gradually come to better understand the person, spend more time with the person, share more memories with them & they become a close friend, that's wonderful! I think friendships, even the totally short ones that you know won't last~ like the girl you talked to at the line for the roller coaster at Six Flags or the kid that was sitting across from you in the subway when you weren't sure where to get off, are important too! Because you, as a person, as who you are, can leave yourself in that person's memory. The kind of impression you give them, the kind of feeling they get from talking to you - even if it's temporary, a kind of warmth or an energy, or a admiration, that kind of thing is like a small good deed. :]
&as for best friends, this is how my layout goes:
aquaintances - like classmates or neighbors that you talk to often.
friends - sometimes the phone, occasionally we go out.
best friends - the kind of friends you call when you want to have fun or the ones who are all over your facebook wall
biffles - the kind of friends that aren't friends, but family
Because I am generally the type of person that doesn't hide anything, I tend to make friends with everyone and trust anyone and everyone. If you ask me something personal, it's true that I might have reasons to not tell you, but if you honestly want to know, I will say. Not because I want to spread things, but because I want to be open with people, honest with them. As long as there is mutual respect, trust, and good feelings between a person & I, we will be friends for the most part.