How long ...has this been going on

Today I was speaking to someone who is divorced from his wife of 25 years, and has been for nearly 10 years. He has since, quite recently, married another woman - who he loves. During the course of our conversation it was obvious he still felt very bitter about his ex - the mother of his now adult children. They don't have even an affable relationship; just tolerating each other on occasions when they are 'forced' to meet (eg. graduation ceremony). I questioned how this could be, after all this time - why? What do you think ...

So, what is the answer - or maybe there just isn't one. Any others out there feel they'd have a relevant comment or view.