Friendship in early life is largely produced by proximity and shared circumstance. You are in the same classroom, the same dormitory, the same office for long enough, and friendship develops as a byproduct of repeated contact. What you do not notice, while it is happening, is that the classroom and the dormitory and the office are all versions of the same thing: a shared context, with shared tasks, that gives people something to be alongside each other in. The friendship is not produced by the circumstance alone — plenty of people share circumstances without becoming friends — but the circumstance creates the conditions. Remove the circumstances and the conditions must be replaced with something else, or the friendship relies on contact-for-its-own-sake, which is harder to sustain.
The shared project is one of the most reliable adult substitutes for the shared circumstance that early life provided. It is not the only substitute, but it has particular properties that make it effective. A project has a purpose external to the people doing it, which means that the relationship does not have to bear the entire weight of the contact. Two friends working on something together have a legitimate reason to be in each other's presence that does not require either party to initiate purely social contact. For people who find the maintenance of purely social contact effortful — who are not natural callers or texters but who engage easily when there is something to engage with — the shared project is a delivery mechanism for the relationship.
What qualifies as a shared project in this sense is deliberately broad. It can be literally a project: two friends renovating a room together, building a website for a cause they both care about, co-authoring something. It can be more open-ended: a book discussion maintained between two people, a shared investment in each other's creative work — you read my drafts, I read yours — or a long-running collaborative inquiry into a subject both find compelling. The key feature is not the formality of the project but the ongoing productive orientation: there is always something to do, something to discuss, something to bring to the next interaction. The project generates the contact.
The quality of contact that a shared project generates differs from purely social contact in a way that many people find generative rather than lesser. The shared project brings out a different register of each person. You see how your friend thinks under pressure, how she handles disagreement, what energizes her and what depletes her, where her judgment is sharp and where it needs checking. This is a kind of knowledge that social contact alone — dinners, calls, trips — rarely produces. The project creates conditions of mild adversity and real stakes that reveal character in ways that comfortable social occasions do not. This is not an argument for friendship-as-perpetual-collaboration; it is an observation that the people who have worked on something together often know each other better for it.
The project also provides a structure for reconnection after gaps. If you and a close friend have fallen out of regular contact — a common adult experience, carrying no particular meaning about the state of the friendship — re-engagement is easier when there is a project to re-enter than when pure social contact must be reinvented from scratch. "I've been thinking about the project and wanted to share this" is a lower-friction re-engagement move than "we haven't talked in a while and I wanted to check in," even though the underlying intent may be identical. The project provides the bridge.
The shared project should not be treated as the only form of the friendship. Projects end; what happens to the friendship when the project is complete is a real question. The answer is usually that the friendship either has enough depth to persist without the project — which means the project was doing what it needed to do, which is produce the contact through which depth accumulated — or it does not, in which case the project was more a working relationship than a friendship, which is fine and worth calling accurately. The shared project as friendship glue works when the project is the medium through which a genuine friendship develops and is maintained. It does not work as a substitute for genuine regard.