From what I’ve seen and experienced myself, yes it is hard work. People live so long now, to be with the same person every single day for the majority of your adult life (could be 50+ years) it’s such a long time and a big commitment. And considering how much people can change over the course of a few years never mind their entire life, their beliefs, priorities, interests etc it’s no wonder so many couples outgrow each other or get bored.
Friendships drift apart all the time as people grow and it is seen as normal, yet we are expected to be in a marriage until death us do part?
As a PP said, I don’t think it is particularly natural state for us to be in. There are genuinely happy marriages out there that last the course but I think they are in the minority.