Why do small habits in a partner become more annoying over time?
Because two opposite processes run at once, and irritation happens to be governed by the one that goes the wrong way.
Habituation is the familiar effect: repeated exposure to a stimulus reduces your response to it. This is why you stop noticing a clock ticking. Most experiences follow this pattern.
Sensitisation is the opposite: repeated exposure increases your response. It tends to occur with stimuli that are aversive, unpredictable or perceived as meaningful — and low-level interpersonal irritants often qualify on all three counts. Each repetition adds to an accumulating record rather than fading.
Why partner habits in particular sensitise:
They become interpretable. The first time someone leaves a cupboard door open it is an oversight. The two-hundredth time, it stops being about cupboards and becomes evidence — of inattention, of not listening, of your preferences not mattering. The irritation attaches to the meaning, not the door.
Repetition after being raised. If you have mentioned it and it continues, the behaviour now carries an additional signal, which is a much stronger provocation than the behaviour alone.
Unpredictability sustains attention. A habit that happens sometimes keeps you monitoring for it in a way a constant one does not.
Reduced tolerance under stress. The same habit is genuinely more irritating when you are tired, busy or already in conflict — irritation reflects your state as much as their behaviour.
Negative sentiment override, a concept from Gottman's work: once a relationship's overall emotional balance tips negative, neutral behaviour starts being read as hostile. At that point fixing the specific habit does not help, because the habit was never really the issue.
What helps: raise it as a specific request early rather than accumulating; distinguish genuine preferences from things you are tolerating badly; and notice when the ratio of pleasant to unpleasant interaction has slipped.