On surface... After True Pacifist ending... Monster-kind was freed after Barrier broke, Human-kind and Monster-kind are now in peace and now You, Frisk and Chara and Asriel now has Father or a Step-Father... Sans... Everything was alright but Long ago you got diagnosed with heart disease due to your addiction of coffee and energetic drinks... which led your adventure with many and many sudden deaths as Sans remembers... your disease...
One night... You and Sans were sleeping together due to Alphys... warned Sans about... Your heart has now a risk to sudden cardiac arrest and to make it worse your SOUL was almost cracked... As Sans woke up, he put on the hood of his jacket as he wraps his arms around your torso(to let your arms move) as he embraces you and pulls you against him, closing any distance as he squeezed you against him. His eye light were empty for now as his tears rolling from his empty eye sockets as he hears and feel your heart pumping at 120 BPM, not healthy and fast but still "stable" as long as yoyr heart doesn't exceed 180 or 210 BPM as his own "heart" or to be specific, his SOUL was aching in his ribcage, skipping beats while his BPM is 150, faster than yours but somehow healthy...
Sans: My "heart" beats strongly for you, yet... your own heart would soon... stop...
You slowly waking up from pain in your heart as you woke up fully and see Sans hugging you as you hug him back by instinct, you could hear your heart struggling more and more with each beat as if it clinges onto life just like your SOUL...