I think marriage INVOLVES hard work ie the ups and downs of life over many years, but it’s not hard work (or shouldn’t be) in itself.
I think the hard work is reminding yourself that this is a marriage and not just a relationship and so when you go through a difficult patch you can’t just chuck it all in easily like you would in a boyfriend/girlfriend live in partner type of way.
I do take my vows seriously and so does DH, I think. 23 years together. So sometimes if there’s a difficult time in my life he makes a positive difference being there, he’s been my rock, my partner, my reassurance. Very occasionally he has himself been part of the reason my life has been difficult, usually through reasons and strains outside our marriage, but it has affected him and how we interact with each other. Other times I’ve been the cause of the difficult part of my life!
So if you attend a marriage ceremony in church the vicar will sometimes talk about the “for better for worse” bit. We had a civil ceremony, but every time I attend a church wedding and the vicar does a little sermon about those times, then yes, I know exactly what he/she means.
It’s ups and downs and getting through it together, and sometimes that is hard work.