They allow each other to grow into different people

The person you commit to at 28 is not the person who will show up at 35, or 45, or 55. The interests shift. The values deepen or evolve. The career changes direction. The relationship to family, to ambition, to what constitutes a good day — all of it moves over a decade in ways that were impossible to predict at the start. Long-lasting couples build their relationship on an implicit understanding that this is not just acceptable but expected. They don't punish each other for growing. The alternative — holding your partner to the contract of who they were when you first made it — is one of the quietest and most common forms of relational damage. It makes change feel like a threat rather than a natural feature of a long life. The couples who go the distance tend to have developed an instinct for renegotiating quietly and continuously: adjusting to the person who exists now rather than clinging to the person who existed before. Loving someone across their evolution is one of the most demanding and most beautiful things a relationship can be asked to do.