Why was Beth teary eyed when Dean walked in on her and Dean in season 4 epi 10 and then upset at home in bed explaining it to him again? WHY? Ugh!
Well, because Beth doesn't feel the same way about Dean that most of the fandom feels about Dean. She's certainly not in love with him, and on the whole, she's largely indifferent to him, but she's never relished hurting him, and in fact, doing so causes her to feel extreme guilt—and him catching her with Rio in that moment does hurt him.
This is a man that shot Dean in his own home and who his wife went on to have an affair with, one that wasn't just physical, but emotionally deep and significant to her in a way his own affairs weren't to him. One of the casualties of that affair was that the family-owned business that he inherited from his father was seized and lost to him, and after that, Beth lied to him and pretended that Rio wasn't involved in Boland Bubbles. And Dean only discovered that lie while in prison, after being arrested for their crimes.
Then Beth promises Dean that she's working on putting Rio away in order to free Dean—who is, remember, innocent!—and she tells him she does, only a few days later, Dean walks in, and finds his wife sitting down with this same man cooking up their next scheme. In that moment, too, she chooses Rio over Dean, asking Dean to leave.
Dean is a bad husband. He's a bad husband from the start of the show, and he never does grow to be a good one. Part of Dean's badness, though, is emphasized by the way that Beth has been a good and loyal wife to him up until the discovery of the affairs and the lost money. At that point, she becomes a scorned and angry wife, and she throws him out, and it feels like justice to her and the audience because at that point, he's reaping what he himself sowed.
But over time in the series, Beth becomes a pretty bad wife, too. She lies and keeps secrets from him. She uses him for sex that isn't about him, but about the same man with whom she cheated on him. She is dismissive of him, infantalizes him, and really only stays with him for the selfish reason of using him as a crutch for normalcy and a built-in babysitter. By the end of the series, at their best, all she does is placate him. The only moments of true joy they share together are when he's unknowingly done something to serve her business interests with Rio. Their relationship is hollow, their foundation is deeply cracked, and their marriage is only intact because Beth has a fantasy about a normal life which she keeps as a lifeboat to weather the tumultuous storm of her relationship with Rio.
And this moment in the series comes right after Stan tells Beth in 4.09 that she doesn't care about anyone but herself, that she "use[s] whoever [she] can to get whatever [she] want[s]," something we see her reckoning with after she steals the money back from Phoebe's evidence stash and she tells Rio she was, "Just being herself."
She's teary-eyed and upset at the start of 4.10 because she knows she's doing something wrong and treating someone badly—a man she built a family and a life with, a life which she thinks she wants to and goes through the motions to save as they keep talking about how they have a "chance" if they go to Nevada.
Ultimately, that's all a pipe dream, but Beth's recognition of that her treatment of Dean being wrong is something that keeps her morally gray. Dean certainly doesn't deserve Beth's forgiveness, nor for her to be the good wife to him that she once was. But there was a point at which it would've been kinder if they let each other go. This scene was a stepping stone to get them to do just that in the finale.



















