Though some people don't view being non-monogamous as that big of a deal and don't believe that their relationship will suffer if either they or their partner cheats, for others, finding out that their partner cheated on them can feel like all of the air is being sucked out of the room. Lots of people wonder why others cheat, and if your partner cheated on you, you might be especially curious as to their reasoning. But there are actually changes that happen in a man's brain when he cheats (or before he cheats) that might explain what's going on. That being said, in science, there's a difference between correlation and causation, so just because your partner has some of these things or experiences some of these things doesn't necessarily mean that he's definitely going to cheat, it just might make him more susceptible to doing so.
When you find out that your partner has cheated, however you may define it, it's not unusual to feel at least a smidgen of temporary betrayal at some point. You also might have a lot of questions about their motivation and reasoning, how it all went down, why that person, and more. Though you and your partner may or may not ever talk in detail about those things, and though your relationship might not make it, understanding some of what might be going on in their brain before and when your partner cheats is still something about which you may want to know more. Even if it only serves to satisfy your own curiosity.