Why is the first year of marriage the hardest?

So me and my husband have gotten married 5 months ago. (We have been together for 5,5 years, living together for 4 years)

But everybody keeps on telling us that the first year of marriage is the hardest. And they say that most divorces happen within the first year of marriage. I can’t help but wonder why that is.

So please give me your insight/ thoughts/ experiences/…