> I remember hearing years ago that the ingredients for a strong friendship are (1) serendipitous, unplanned encounters, (2) vulnerability, (3) sustained time spent together. Housemates seem like the only modern avenue to this for adults

The ingredients for a strong friendship are doing things together without forcing it and contact surviving when things change (e.g. geographical move, shared activity ending, family change).

The fact that you needed such a constant school like, unavoidable presence speaks to your own lack of sustained activities and communication in other areas. You haven't made friends through sports, hobbies, shared courses or even work? Only by 24/7 living with them?

You may want to change that. It may seem like it requires more effort at first but it's not that different and lasts beyond the see each other every day phase.

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.