In the linked article, just refer to earlier posts. Only going on and on about an ex is really "bad." And it's more rude than anything. To bring this person into conversation is only to compare you to them, and that's fine once or twice, as long as you win. Beyond that, you'll start to wonder if they're still hung up on them.
That said, as above, the reasons for dumping a woman? Lying, almost at all, and cheating. Those are gimmes. But it's possible for two people to just grow apart. I had one relationship that was going fine on that "new relationship" energy, and then one day I realized she just had zero inclination to "do better." It wasn't that she was in a bad place either; just a lack of drive.
Really, it's probably different for every guy, depending on what he values.