Years later, there is less unknown. In exchange, there is safety, history, closeness and extremely detailed knowledge of each other.
That creates an interesting tension. Predictability is wonderful for attachment. Erotic curiosity sometimes thrives where something in the other person still feels not fully possessed or completely known.
That does not mean couples need to become cold or mysteriously disappear for three days in order to preserve desire. Sometimes it is enough to see a familiar person outside their usual role: with friends, performing, in a new job, travelling, or simply deeply absorbed in something that has nothing to do with us.
It is very easy to stop noticing someone when we feel we already know everything about them. But knowing someone for a long time is not the same as knowing everything they can still become. Sometimes that restores interest far more effectively than trying to reproduce the first year of the relationship.