Why do we say we know someone once we've seen and met them on daily basis? Do continuous encounters and just occasional conversations enough for one to claim to have known another?

Had the world not made us be too cautious and suspect every strangers of being ill-willed, I'd gladly throw myself to any random strangers I'd feel like talking to. I'd rather just meet someone once and no more, but just talk whatever we feel like talking. A lengthy talk. It doesn't have to be too deep or thoughtful; sometimes you can get flashes of inspiration from a light pointless conversation.
I wouldn't have to know the name, or phone number or whatsoever, we may or may not meet again but I'd walk away feeling I've already known him more than half the people I've met on daily basis in my life.