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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Beep... Beep... Beep... 

That constant sound, it still continued on all throughout the entire time Sans was sitting at Frisk's side. Of course, that was a good thing, it meant that she was still living, that he hadn't lost her just yet. But God damn, could it drive a person crazy, just hearing that noise over and over. It would be more reassuring if Frisk was actually awake. The continuous beeping was only made a little comforting, but not completely. Why couldn't she just wake up? Sans had just been sitting there, holding onto her hand, and for a good while, he had just kept muttering under his breath how he wanted her to wake up.

He wondered how Leo and Luna were doing with Toriel. He admittedly felt somewhat guilty not coming home to see them, and just dumping them on the goat monster, (even if she had offered to take care of them). He just knew that he could only keep up a lie so long of what actually happened to Frisk, and if she didn't come home... What was he supposed to tell them? He couldn't actually tell them that she died, could he? Such thoughts were stressing him out, as he was laying his head on the edge of Frisk's bed, next to his hand clinging onto hers. He was so damn exhausted. That wasn't made any easier by that constant noise that just echoed over... And over. 

Beep... Beep... Beep... Beeeeeeep...

At that moment, Sans sat upright, as for a second, his expression contorted into a look of confusion. Wait, why did the noise change? On the heart monitor, why was there just a straight line? He was so tired he was asking himself stupid questions, until it struck him. That sound... Did it mean that her heart had stopped?! "w-wait-- frisk?!" Sans said in a moment of panic, before he shot up out of his chair and he looked desperately towards the door. "DAD!" Sans had screamed at the top of his voice. But no one came. Why was no one coming?! The skeleton urgently looked back to the girl he loved, gripping onto her hand tightly as he felt himself beginning to shake. "c'mon, frisk, dammit! don't do this to me! please!" Sans begged her, with tears streaming down his cheekbones. 

But she wasn't answering him. 

"FRIIISK!" Sans screeched at her. 

And then he woke up. He jolted, finding himself still laying at Frisk's side, with his head on the bed beside her figure. He could still hear that beep... beep... And he could only let out a sigh of relief. It had been just a terrible nightmare. He should have guessed. Frisk would never give up on him like that, would she? "You've been crying." A voice said, which caused the skeleton to stiffen. Even more so when he felt someone place a hand on his skull, and he slowly went to sit up, only for his sockets to widen in shock. It was... Frisk... She was... She was awake! She was awake and smiling warmly at him, almost as if he hadn't been sitting at her bedside for days on end panicking over her well being. Not knowing what else to do, he shot out of his seat and hugged her tight. 

"ya fuckin' idiot!" Sans growled, as he was really starting to cry. He heard Frisk let out a weak giggle as she mentioned for him to not hug her so tight, but he couldn't help it. He was just so fucking glad that she was okay, that she wasn't dead like what had happened in that nightmare. He just ended up kissing her on the forehead, then on the cheek, and then of course he had to kiss her passionately on the lips. "ya should have fuckin' listened to me! why didn't ya listen?!" Sans yelled at her, only to exhale and hold her. "...ya frightened me... i thought i was gonna lose ya."

"I guess we're even now then." Frisk teased, only to begin frowning. "I didn't expect that... Asshole to overpower me like that. I was scared, admittedly." She murmured, glancing elsewhere as she slightly gripped the blanket that rested upon her body as her brows furrowed a little. "I just didn't want to let him get away. He's one of the people who killed my parents, Sans. I doubt you would easily let anyone go if they killed Gaster." Frisk said, which made Sans' pupils trail towards the ground, as he knew how true that was. If anyone hurt his dad-- Heck, when his dad had been taken from him, and he found the people who did it, he just wanted to kill them all. He still would do that kinda thing in such a situation. 

"...you're not gonna still try and go after him, are ya?" Sans asked, and when Frisk looked as if she had been caught stealing candy from a candy jar by their parents, Sans gave her a look of disbelief. "ya can't be fuckin' serious. are ya braindead?" He knew what he was saying probably made him sound like a complete dickhead right now, however, didn't he have the right to be upset? Was Frisk seriously thinking that despite almost getting murdered, she was still going to try and go after that guy? He could take care of him for her, or someone else could. She didn't have to do it. 

"No, I'm not 'braindead', Sans." Frisk shot back, clearly a bit upset by his harsh words. "But this is important to me!" 

"what happened to the girl who didn't like killin', huh? who thought it was against her morals? ya don't have to do this! i could do it. gaster could do it. literally anyone else could do it." Sans ranted, as he narrowed his eyes towards Frisk, who was crossing her arms over her chest with that frown still plastered across her face. She put in the argument that it was her fight, because it was for her parents, and when she said that, Sans could only sigh and shake his head, thinking she was being completely ridiculous. 

"okay. what happens when he almost manages to kill ya again. no. say, next time, ya do fuckin' die. what do ya think that is gonna do to everyone else?! to me?! what about leo and luna?!" Sans seethed, as he stood from his chair as if to further express the point he was trying to make, because he was clearly passionate about what he was talking about. He just didn't want to lose Frisk, and he was scared that he was already losing her in a whole different way. She had just become so used to killing people like he had that... She was becoming desensitised to it all, just like he had become. And he really didn't like that about her, because he didn't want her to be like him. He didn't want anyone to be like him. 

"do ya know how hard it was havin' to lie to them, and tell them ya were okay when really ya were lyin' unconscious, with the possibility of dyin'?! do ya think it was easy for me to see the look on leo's face when it seemed like he didn't really believe me?!" Sans complained. "...it's been hard... without ya, it's been really hard. and i don't wanna know what it would be like if i permanently had to live like that because ya did somethin' stupid..." Sans finished, as he went to slowly sit down again, and he was blinking back tears at the thought. He just wanted to keep protecting her, that's all he had ever wanted to do. Frisk would never stop being the girl he wanted to keep safe from the terrible things in this world. He didn't want her to think he thought of her as weak, but... It just wasn't safe for her. 

"Sans..." Frisk started. She paused, and reached out for his hand, and seeing her do that, he decided to give her his hand to hold onto - the hand that had the ring on it that she gave to him. She weakly smiled at the ring, and he could only give a smaller smile in return, because right now he didn't really know what to feel. He was glad that she was awake, but he was upset because she was talking nonsense. "I know you're worried about me... And I know that what happened was crazy. But this... This means a lot to me, okay? I've been haunted by the death of my parents all my life, even if it has become easier to deal with over the years. But putting him behind bars isn't just going to take care of him. If I have the chance to get rid of him, and to stop others from getting hurt too, then I want to be that person." 

"Don't think that I just think your feelings aren't important. I'm sorry that I haven't been there to help, and I'm sorry you've been having to deal with things all alone. But please, I don't like you thinking of me as some weak, frail girl who can't take care of herself. Who will only run away because she can't deal with something. That may have been who I was when I was 19, but that isn't who I am anymore." Frisk softly told him, gently squeezing his hand. Sans didn't really know what to say to all of that. He understood that she didn't like to be seen as someone who couldn't take care of herself anymore. But... He just... He wanted her to be okay. That was all. However, no matter how much he argued with her on this subject, he couldn't seem to make her budge. 

"I will make a compromise though." Frisk added, causing the skeleton to raise his eyebrows at her a little. "If you let me do this, if you let me take care of him, after it's done - I'll quit." She said with a small smile. Sans blinked at her in shock. "I promise." She beamed, squeezing his hand once more. Hearing her say those two words did bring a small grin to his own face, as he wasn't really one to like making promises himself. But he knew that when they were made, they meant a great deal to him. If she really would keep her promise and make that compromise, then... 

"okay..." He agreed. 

The skeleton would allow her to go on a mission, one last time.