I think it's more about compatibility. We all change so much throughout our 20s and 30s and the likelihood of actually still getting on well with a partner you met 10-20 years ago after all the stress of daily life, children, work etc means that lots of relationships fail. This doesn't mean either party is somehow 'faulty' or deficient in some way, just that they are not suited to each other any more.
My ex was not very emotional, not involved in family life and very focussed on his work. For me, this was a deal breaker. His new GF is in the same line of work, also very passionate about it, has no DCs so doesn't demand family time with him and is actually even more unemotional than him!
I was always too needy and too emotional for him, but my now DP is also a very emotional person and we get on brilliantly, we are both as needy as each other! He used to be very self-absorbed and didn't treat his ex partner very well. She had an affair and left him. With me he is totally different, very generous and loving and has learned the error of his ways (plus I am obviously fabulous and more deserving of his lovely treatment
)
His ex continued to smoke after he gave up, which pissed him off daily, she was also still living the party lifestyle of her 20s while he was more settled into family life, so that was another stumbling block for them. What worked for them when they met was no longer a match in their 30s.
So what I'm saying is that for each of us, our previous partners didn't really do it any more, but that makes us just perfect for someone else. Don't despair, one man's trash is another's treasure!