I've been thinking lately about the things you need to know about another person before you feel like you know who they are. How much do you need to know? How quickly can you find it out? What makes you feel comfortable?
I can think of three couples off the top of my head who went from single to married in well under a year, or are in that process now. Meanwhile I have friends I've known for two decades who can still surprise me.
I guess it boils down to what you need to know to bond in friendship with someone, or to become a couple or to choose them as a roommate. And how do you find out what you need to know? Time is the only way I've ever found, but that is not necessary for everyone.
Communications theory tells us we're likely to share with people when they share with us. Maybe some are willing to do readily while some of us are far more reticent. We humans are fasinating creatures in our differences.