A question as old as can be.
The newness of the relationship wears out for most, but the love doesn't wear out for all.
Most people who are not at each other's throats over issues can keep the love. But they
do lose that "spark" of excitement in most cases.
Changing something helps. When the kids are grown and gone, vacations might keep
a couple going, or a shared hobby such as golf or sailing or wine-tasting or whatever they
want to try. Also, major remodeling of the home or their life keeps a spark going.
Some just get more used to each other and don't need the spark anymore.
Sorry, the answer is all over the map. Some need that spark and feel they lost it and
see a new conquest walking down the street. Many people like new stuff, so they ditch
the old. What a shame.
What helps me is not to put my standards on others. Some married (or became partners)
for their own reasons- security, pregnancy, loneliness, sex, excitement. Who are others
to tell them they must stay together? Although, if you really loved the other, you would do
your best to communicate your feelings with them and maybe go to therapy together before
just walking away. Still, if that were done, maybe they both wanted to split, or maybe one
never loved the other.