For those who are divorced, what is the main cause of your failed marriage? Do you regret it?

I knew a guy who after 15 years of marriage just woke up one morning and said to his wife.
I'm getting a divorce.
He couldn’t really say why, he just said he realized he didn't love her anymore.
I think all his annoyances and frustrations just built up over time.
It seemed really out of the blue.