Why you should stop asking "why"...and instead ask "why bother"

"Men don't cheat because of who she is, they cheat because of who they're not."
~Charles J. Orlando, author of "The Problem with Women...Is Men"

We often talk on this site about the question of "why". It's generally the first word that forms in our brain when we learn of a spouse's affair. But sadly, we often answer that question with a catalogue of our own perceived failings: I've been preoccupied with the kids; I've been busy with work; I've been stressed with moving my parents into a nursing home; I'm aging; I'm fat; and blahdy blah self-flagellating blah.
It's all, of course, bullshit. So is all the stuff we tell ourselves about what she has that we don't. As my husband's therapist once said to me, "what she's got, you don't want."
No matter what your spouse tells you or what you tell yourself, he cheated because opportunity met moral failing and wound up in bed together. That's not to say that your list of "why"s aren't necessarily true. Maybe your marriage was under strain. Maybe you could have spent a bit more time at the gym. Maybe you did take your stress out on your husband. All of which are absolutely valid reasons for your husband to suggest counselling, or anger management, or even a separation. They're not valid reasons for cheating. I'm not sure there is a valid reason for cheating. 
The time will come when the two of you, should you choose to rebuild your marriage, to pore over your marriage like a couple of forensic detectives, looking for just where it went off the rails. Ideally you'll do this within the context of "where can we improve our communication so neither of us feels so alone again" rather than "this is the long list of ways in which you're a complete asshole". But sometimes that compassion and willingness to be open to your cheating husband's pain takes time.
It also takes strength, which doesn't come from beating yourself up about the myriad ways in which you somehow brought your spouse's cheating on.
Your task, post-betrayal, is to keep yourself strong. No easy task. It means extreme self-care – avoiding anyone who isn't loyal to you; it means avoiding any commitment that makes you feel more vulnerable; it means eating and sleeping; it means avoiding excess (or any!) alcohol, drugs, gambling, shopping or other means of numbing your feelings. And it means stopping, at least for now, asking "why".
Your husband likely can't tell you. Not really. People who cheat aren't generally the most self-aware. They can learn self-awareness and the fallout from cheating often spurs them in that direction. Decent people who cheat are often so disgusted with themselves that they want to know how they were able to do such a thing in order to ensure they'll never do it again. But there are plenty of guys equally disgusted with themselves who simply can't admit that – it's far easier to blame something outside of themselves (your work schedule; their boss) than own up to their moral failing.
The first group generally make rebuilding a marriage as easy as it can be (which, frankly, still isn't easy); the second make it a whole lot harder and should prompt you to ask whether or not it's worth trying. Without a clear understanding of how people can use other people to avoid feeling pain or shame or loneliness or stress, there's little to prevent them from doing it again.
As Charles J. Orlando points out in the quote above, men cheat because of what's missing in them, because of who they're not. Who they're not is a guy who recognizes when he's seeking escape in an unhealthy way. Who they're not is a guy who recognizes the damage created by cheating before he does it.
Instead of asking why he cheated, the question you should be asking is why – and if – he deserves the chance to rebuild your marriage. It's the question he should be asking himself too.