Why do we remember exes more fondly than they deserved?
Because memory is reconstructive rather than a recording, and several well-documented biases pull recollection in a positive direction over time.
The main mechanisms:
Fading affect bias. The best-supported explanation. Research consistently finds that the emotional intensity attached to negative memories fades faster than that attached to positive ones. The events are still remembered; the feeling drains out of the bad ones more quickly. Over months and years this leaves a systematically rosier balance without anything being forgotten.
Rosy retrospection, the broader tendency to evaluate past experiences more positively in hindsight than at the time — documented in studies where people rated holidays and trips more highly afterwards than during.
The peak-end rule. We judge experiences disproportionately by their most intense moment and their ending, rather than by the average. A relationship with brilliant highs is weighted heavily by those peaks.
Selective retrieval. Memories are cued by context — a song, a place, a season. Those cues are frequently attached to good times, because those are the occasions that produced distinctive memories. Ordinary daily friction produces few retrieval cues.
Comparison against the present. Loneliness, or a poor current situation, changes the standard you compare against. A relationship that was mediocre can seem preferable to being alone, which is a judgement about now rather than then.
Idealisation in absence. Without ongoing contact, the person is remembered as a summary rather than encountered as a full, irritating human being. You are comparing an actual person to an edited version.
Why it matters practically. This bias drives returning to relationships that ended for sound reasons, and it explains why people are frequently surprised by how quickly the same problems reappear.
A useful counter: write down the specific reasons at the time it ends, factually, and read them later. Contemporaneous notes are not subject to the bias that your recollection is.