It started as a simple headache; a dull, throbbing pain in the back of his head. It was nothing he couldn't handle.
Take some medicine. Drink more water. Relax.
Medicine didn't make it go away, it only lessened the pain. Still, he shook it off and put it off to the side. He had more important things to tend to.
He ignored it. He forgot it was there. He got headaches all the time. He continued on with his life. Nothing was going to keep him down. Papyrus needed him to man his station. After the human left the underground, he was left feeling... upset? No. More like.. heartbroken. No, that wasn't it either! He felt like... like he'd just been punched in the soul. His soul physically ached. He grew attached to that human, and they seemingly didn't even care.
The pain gradually grew in intensity. Not enough to notice, at first. It took two weeks for him to notice anything. He continued to take medicine, drink water and relax. But it began to bother him. Like something chipping away