To clarify, the frequent encounters of a housemate situation hasn't been required to sustain friendships for me, but instead to get "escape velocity," if you will. The great friends I made of former housemates still keep in frequent contact with via phone calls, etc.

You said "doing things together without forcing it" is important. That's precisely what I mean with housemates. In the storm of people's busy lives and moods, living with someone just provides so much more effortless surface area for bonding than pre-arranged, formal events to do sports or a hobby.