Sans is a MONSTER and not a HUMAN, so he doesn't follow HUMAN biology like a HUMAN skeleton. Same with Gaster and Papyrus.  

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"are ya fuckin' insane?" Was it really any surprise that those were the first couple of words that came out of Sans' mouth to what Frisk had said? No, probably not. Frisk surely knew how time consuming Asgore's job must have been, right? How stressful it must have been. There was surely, absolutely no way in hell, that she was even considering such a thing. He couldn't care less if the Mafia closed down if it meant that Frisk could avoid this. But when she gave him that sad look, as if she had absolutely no idea what she had done wrong, he really felt like he was about to lose his nerve. "surely, ya can't be fuckin' serious. asgore, surely, ya can't let her do that." He didn't want it to seem like he was making decisions for her, like he was telling her what to do, but what she was doing right now was stupid. However, the moment he saw a look on Asgore's face as if he was debating it, that was when he really felt like he was going to lose it. Even if Toriel was standing next to her husband looking like she disapproved, somehow, he felt like that wasn't going to stop what might happen. "ya know what? fuck this." 

That being said, Sans decided to turn on his heel, heading right for the elevator. He ignored the fact that Frisk tried to stop him, because right now he was just mad, and he knew that if he turned around for her to talk to him, he might just utterly snap at her. He made his way to the elevator, went up, left Grillby's, got to his car and he ended up kicking his tire in frustration. Frisk couldn't be serious. 

"Frisk, are you sure about this?" Chara asked towards her sister worriedly, seeing the way Sans had walked out back there. It was true what she said, that surely there should be someone else to take Asgore's place, but she couldn't imagine that Frisk would be the one to step forward of all people. Especially when she had Sans. It was the same reason why she couldn't have seen Sans saying he would take such a job, because he had Frisk. 

"Absolutely not. That is crazy." Toriel spoke up. "Do you know how stressful his job is?" 

"Yeah but, mom..." Frisk said. She knew it may have seemed crazy, that she was even thinking of doing this. Clearly, she had bothered Sans by deciding to say what she had said, as he didn't even want to stay to hear any more of the conversation. She didn't want it to seem like she was doing this just so she could stay involved, like she was obsessed, because that isn't what this was. And that's what she would explain to Sans, after the conversation that followed. It was just a lot of back and forth between her and her mom, arguing as to whether or not she could actually do this. Until it was actually decided. 

She was going to do this. 

Part of her wondered whether or not she made the right decision. Especially when she was back on the street, and she found the skeleton, sitting on the kerb, beside the car, seeming to fiddle a little with the ring on his left hand. Then, after a couple of seconds of doing that, he stopped fiddling with his ring to breathe a puff of smoke from the cigarette he had in his mouth. He exhaled. "I thought you didn't smoke anymore." Frisk teased from where she was standing, as Sans glanced over his shoulder to look at her, only to look away and grumble under his breath. 

"i didn't ever feel the need to, until just now." Except rather than all those cigars he used to smoke, he just decided to head to the nearest store and buy a pack of cigarettes. It was better than nothing. "what brought ya out here? i thought ya would be inside havin' more fun bein' their boss." He growled, glaring down towards the concrete as he heard the sound of Frisk's footsteps edging closer to him, and then she appeared to sit down next to him on the edge of the sidewalk. He couldn't even look at her, because if he did... 

"Sans..." 

"no. fuck. off." Sans seethed, not sure if he had ever found himself this angry in awhile, and at Frisk of all people. Normally, he didn't like to snap at her. If he did get furious, it made him look like the prick, because Frisk was the innocent angel who couldn't possibly do anything wrong. Again, he took a puff of his cigarette and he went to stand up, so he could throw the cigarette down and crush it under his foot, before he gazed down at Frisk, and he saw her giving him that offended look. 

"don't give me that look. like it's a big fuckin' shocker that i'm upset. ya said ya were gonna quit, and what do ya do? ya run straight back to it again!" Sans yelled at her, while he clenched his fists at his sides. "am i just not that important to ya anymore...? is that it? are our kids just not that important to ya anymore?!" He went on, just feeling like he needed to get all of this out, because if he didn't, he was going to explode. Not that he wasn't already exploding already, due to the fact that Frisk was going against her promise that she had made, when she said she would quit. He didn't think that would mean she would quit, and then come running straight back towards the dangerous thing he was trying to keep her away from. 

"Sans, you know that isn't what this is..." 

"then what is it?! because as far as i can tell, that's exactly what it fuckin' is. no matter how many times i tell ya to just stop, ya don't listen. as if what i say doesn't mean shit to ya." 

"Look, the reason I said I would do this is because... You saw how worried everyone looked. How freaked out Chara seemed. I just wanted... I just wanted to help everyone." Frisk shrugged, having a small expression of guilt plastered across her face, while the fierce look on Sans' face slowly softened a tiny bit. He still looked pretty snappy, but less so than just a small second before. "This isn't about you Sans. And I don't mean that in a way as if to say I don't care about you. I mean that in a way that says: you still mean everything to me, and so do the kids, and I'll try my best not to let this take up too much of my time. But it was just something I had to do." She told him gently, as she stood up to be more level with him, and she went to reach for one of his hands. 

The skeleton appeared hesitant when she did this, but with a small exhale of air, Sans allowed his hand to slip into hers and she gave it a small squeeze, while she weakly smiled at him. He tried to give a small smile in return, but it was more sad than happy. "i just... ya mean everythin' to me. i can't ever lose ya." He told her, stepping closer to her, only to put his arms around her to hold her. "i love ya, frisk... so please, don't make me regret it." He told her quietly. He didn't want this to turn into another situation where he felt like a fool for loving someone who was only going to change in the end.

Again, it wasn't really Blair's fault that his first relationship had ended, but it still had had an effect over him, emotionally, and for a long time, before he knew Blair's story, it had damaged him so bad to have just lost her like that. If he lost Frisk, well... It might just be the biggest regret of his life, and part of him questioned what he would even do after that. Especially with kids in the picture, he also wanted to think about them, and how they might be affected. For example, Leo and Luna would have that play soon that they had to perform, and Sans didn't want Frisk to miss that, because if she did, it might just be the last straw for Sans, after the multiple times he had complained at her, and tried to get her to listen. If he even had to see that look of disappointment on Leo or Luna's faces, then... 

"I won't..." Frisk promised. 

"well, then... good."