The reason a habit needs a finishing signal

Read more is not a habit; it is a direction. Read until the end of the chapter, then put the book on the shelf, is a habit, because it contains the moment at which the sequence is over. Without that moment the behaviour has no unit, and units are what get counted, remembered and repeated.

What everyone sees

Habit formation is discussed almost entirely in terms of triggers — what starts the behaviour — with the ending treated as a detail. The ending is doing structural work. It defines where one instance stops, which determines whether an instance exists to be recorded at all.

What is actually happening

Wood and Rünger’s review of habit describes it as a learned association between a context cue and a response, formed through repetition of a delimited action in a stable setting, which requires the action to have identifiable boundaries. Zacks, Speer, Swallow, Braver and Reynolds show that experience is parsed into discrete events at boundaries where the situation changes, and that these boundaries organise memory: material is encoded and retrieved in event-sized chunks rather than as a continuous stream. Combined, an unbounded activity is a poor candidate for habit formation on two counts. It supplies no repeated unit for the association to attach to, and it produces no event boundary for the completed instance to be filed under.

Why it stays hidden

The absence hides because an ending feels like an administrative detail while a start feels consequential. Nobody has trouble noticing that they failed to begin; almost nobody notices that they never finished, because a behaviour that trails off does not generate the discomfort that a behaviour never started does. The missing element is the one that produces no complaint.