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

Sorry for the lack of updates recently. 

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Kindergarten wasn't really the same when her brother wasn't here, especially with the way things had been the night before. Despite the fact that things didn't seem like they were going well, Luna was trying her best to keep positive and happy, even with the absence of her little brother, Leo. She was just hoping that he was doing okay, and that their dad was okay too. All she wanted was for everyone to be happy, and it had made the little girl sad when she had heard her mom crying last night, even when she had been put to bed. Honestly, after hearing her parents argue, and seeing Sans walk out with Leo, even as a child, such things made it difficult to sleep at night. 

"Is Leo okay?" Kyle had asked Luna, deciding to sit at the small table with her which she was using to draw. Really, she didn't know if Leo was okay, that's what scared her. If she knew, she could give a truthful answer and say that he was just fine. But after the events of last night, she wasn't really sure of how her younger brother was doing. Their dad was a good dad though, so she was sure he was taking good care of Leo. 

"He's okay."

As okay as he could be. Actually, right about now, as Luna was having this conversation with Leo's newfound friend, Leo was actually laughing, as he was being lifted in the air by his uncle Papyrus, almost the moment he and Sans had gotten through his front door. Sans couldn't help but smile warmly, seeing the two interact with each other. It made him glad to see the biggest smile on Leo's face that he had seen in the last couple of hours. If anyone did a good job at trying to cheer anyone up, it was without a doubt: Papyrus. He was great with people. 

"IT'S GOOD TO SEE YOU!" Papyrus said, after carefully going to put Leo down and Sans gave his brother a small grin, seeing that it was nice to see him too. He tried to make his joy seem as genuine as possible - mainly for Leo, more than Papyrus. "LEO, METTATON IS ACTUALLY IN THE LIVING ROOM, IF YOU WANT TO SEE HIM TOO." Papyrus mentioned to the 6 year old, who suddenly seemed excited once more to see the robot. The tall skeleton beamed, watching Leo run into the living room at full speed, and it was only a couple of seconds later the two brothers could hear the robot and the 6 year old conversing. "THAT SMILE OF YOURS COULD BE A LITTLE BRIGHTER." Papyrus said, only for the look on Sans' face to falter and he exhaled, glancing elsewhere.

Where the hell had his skills  gone, where he was easily able to convince people he was okay when he actually wasn't? Was it just because Papyrus was his brother, so he perhaps knew him too well? Okay, fair enough, Sans wasn't going to keep hiding how he was doing around his brother anyway, as he had come here for a reason: not only to make Leo feel better, but himself too. And he knew so much better now that he didn't have to rely on just himself to try and cope with things. That's why he had friends and family - they were there to help. He just wished that Papyrus wouldn't notice it almost the moment he had stepped through the door. 

"y-yeah... i'm kinda stayin' with dad for a bit, i guess." Sans shrugged, avoiding Papyrus' eye. Though, out of the corner of his vision, it wasn't very difficult to notice the confusion spread across his little brother's face. Sighing, Sans looked back at Papyrus, really starting to frown now. He didn't find much point in fake smiling when Leo wasn't in the room. "me and frisk kinda... split up, i guess..." He croaked, immediately seeing Papyrus' face contort into a look of shock. To tell the truth, it took everything in the older skeleton to not have a break down just talking about this, or seeing his younger brother's reaction, because it was likely going to continue hurting for awhile. Some might say it was his own fault that he was hurting - he was the one who ended the relationship after all. But would he have been happier having Frisk ignore his words of concern, he wasn't sure. Either way, it didn't seem like he'd get a very positive outcome. 

"I DON'T UNDERSTAND, WHAT HAPPENED...?" Papyrus questioned, trying to keep his voice down for the sake of Leo in the other room, who did sound like he was having a blast with the robot, as he could be heard laughing and giggling. "YOU TWO SEEMED VERY HAPPY." He pointed out, knowing he was likely stating the obvious here. But that was the thing, to Papyrus, it made zero sense that they were suddenly apart; they loved each other. So, to hear that they had broken up, it was a real surprise. And not a good one.

"yeah, well..." Sans started before trailing off, not really knowing what to tell him. His thoughts were still completely jumbled, and he had debated on and off whether to go back home and apologise to Frisk. But then he would think, well, I'm not really the one who is mainly in the wrong, am I? It just went back and forth, and he wasn't sure whether to blame himself or not. "i got fed up... i'm sure ya know about frisk bein' the mafia boss by now?" Sans murmured, and at Papyrus' nod, he continued. "...it's just too much. seein' the girl i love constantly changin', and getting herself into trouble; getting herself hurt." Sans quietly said. "...i can't stand the thought of losin' her like that - of her dyin'... i can't stand the thought of havin' to sit at her bed side again, desperately and mentally beggin' for her to wake up..." 

He knew that may have seemed selfish of him, because Frisk had been there for him countless times when he had gotten into trouble, and she had sat at his bedside whenever he needed her. But he wasn't Frisk. He didn't have the patience of a God, (or a Goddess in Frisk's case). He had been hurt too many times to count, that he wasn't sure he could mentally take another strike, because if he had to deal with anything else... He... Let's just say, there would be no happy ending for him. Even though, no longer being a relationship with Frisk, he was already questioning if he was going to be happy ever again. 

Don't get him wrong, he was glad to have Leo. And Luna. Even if his and Frisk's relationship may never be okay again, he would try his hardest  to always be the best father to those two. Their happiness was everything to him, and he never wanted to be part of the reason that they were unhappy. Not only did he feel guilty about that argument with Frisk, because he may have really hurt Frisk's feelings, but also he was worried about badly affecting the  children. He didn't want to put a damper on their lives because he and Frisk were no longer together. 

"COULDN'T YOU TWO JUST... TALK ABOUT IT?" Papyrus asked.

"there's nothin' to talk about, papyrus... i tried... countless times. i just..." Sans exhaled. "i hoped that when i showed her i was done, that maybe that would drill it into her brain but..." He didn't know what else to do. If he just went back to her now, wouldn't she continue with what she was doing, getting involved with the Mafia, despite the fact that he didn't like her to? Would she just continue not listening to him, not taking his feelings into consideration? Because he wanted to think about her point of view. He wanted to see that she was just trying to be a good person. But why did she care so much more about the well being of people that probably wouldn't give so much a shit about her, compared to caring about how he felt? And no, he wasn't talking about Asriel, or Chara, or Andrew, or whatever... Just... People they never really interacted with.

No, it wasn't easy to lose your job. But why was it Frisk's responsibility to make sure that that didn't happen to certain people? It didn't have to be her responsibility. It could be someone else's. It could be the responsibility of the person who had lost their job, and they would just have to try and find a new one. Like himself for example, he technically didn't have a job anymore. He quit the Mafia, didn't he? He did that out of his own choice, and now he was going to have to find something different to do, unless he went back to Frisk and asked for his job back. But was he going to do that? 

No.