When you partner, a thing happens to your friendships that nobody warned you about and that the partnered side of the equation tends to underestimate. The friends who are not partnered begin, slowly, to feel like a different category of person, and the asymmetry that opens between you is one of the quietest and most consequential reorganizations of adult life. It is not that you love them less. It is that your defaults have moved, and the defaults are what friendships actually run on.
You used to be available on a Thursday at nine without checking. Now there is a check, even if the check is silent and even if the answer is yes. You used to text back within an hour. Now your phone is in another room because the two of you are watching a show. You used to absorb the long phone calls about the dating disaster, and you still do, but a small part of you is no longer fully present, because you are no longer running the same operating system. Your single friends notice this before you do. They do not always say so, because the social script around partnering still treats it as an unambiguous win that one is supposed to celebrate, and complaining about losing a friend to a relationship sounds petty even when it is accurate.
What is happening, structurally, is that the single friends were operating on the assumption of high availability and they have lost it without compensation. Meanwhile, you have gained a partner who absorbs much of what they used to absorb, and the deficit on their side is invisible to you because it is not your deficit. This is the engine of the drift. Not malice, not coupledom's natural exclusivity, not the partner's possessiveness, just an asymmetric loss of slack that takes years to surface as an actual problem.
The mistake the partnered side typically makes is assuming that occasional gestures restore the equilibrium. They do not. A dinner once a quarter does not compensate for the loss of a friendship that used to run on continuous low-level contact. What restores it, if anything does, is a deliberate decision to maintain some channel that operates independently of the partnership. A standing phone call. A solo trip annually. A thing that is yours with this friend, not yours-and-partner's with this friend. This sounds like an obvious move and it almost never gets made, because the partnered life has a gravitational pull toward consolidation and the consolidation feels like efficiency rather than loss.
The mistake the single side typically makes is assuming the drift is personal. It usually is not. The partnered friend is not choosing the partner over you in any deliberate sense; the partnered friend is simply living inside a different default architecture and has not noticed how much the architecture has changed what is reachable from where they stand. Reading the drift as rejection accelerates it, because the reading produces a withdrawal on the single side that the partnered side then interprets as the friendship cooling, and the loop runs itself.
There is also the problem of overlapping but non-identical life-stages. Your single friends may still be doing things you used to do: late nights, last-minute travel, the kind of social fluidity that requires no scheduling. Your partnered life may be moving toward mortgages, in-laws, possibly children. The activities that used to be the substrate of the friendship are no longer available to you in the same way, and you have to find new substrates or the friendship will run out of things to do together. This is not anyone's fault; it is the topology of adult life. But it can be navigated, if both sides notice it before it becomes terminal.
Law 3 is the relevant law here because connection is not a constant. It is a flow, and the flow has to be routed somewhere. When you partner, a lot of flow that used to go to friends gets routed to the partner, and unless you are deliberate about leaving channels open elsewhere, the channels silt up. The work, for both sides, is to refuse the easy collapse into a single relational center, not because the partner is insufficient but because a life with only one relational center is fragile in ways you cannot see until something cracks.
Finally, there is the version of this that hurts most: the friend you assumed would be in your life forever, who quietly becomes someone you see at weddings. There is no specific moment when this happens. It accumulates, and one day you realize you have not had an honest conversation with this person in three years, and the formality of the next interaction confirms what has already happened. If you want to prevent this, you have to act before you can see it coming. By the time it is visible, the work to reverse it is much larger than the work to have prevented it.