They didn't know for a long time. That is generally the case- you don't know until you find out, and when you find out, it's too late. Or at least that was what Henry had said at his sons bedside the first time around. Shawn had known, in a way. He hadn't known exactly what, but he knew something was wrong from the start. Maybe he really was psychic, as he'd laughed about softly with Gus who was standing at his bedside the second time and every time after that. He probably should have told them was his big revelation the fourth and final time.
The Spencer boy had never been the same as the rest of the kids. In more than the obvious ways. Yes, he had just a dad, and he'd once been taught how to escape from the back of a car. But to him, that was nothing strange. The weird thing was in his head. It hurt more than the other kids did. But he'd never mentioned it. His dad had taught him to be brave. While most of his attempts to shape Shawn into a good cop had been futile, he'd always listened on that. He was 9 when he was diagnosed. He missed most of fourth and 5th grade, going from treatment to treatment and for the majority of the time, living in a small grey hospital room with plain old curtains over the one little window. On his eleventh birthday they celebrated, clinking a Coca-cola can to a beer can and drinking to their little miracle. When he relapsed in 7th grade, only a little over a year from what the doctors had believed to be a miracle, they let Gus in on the little secret, and the endless days of trials and tests seemed a little less lonely with his best friend by his side. And on his 15th birthday, three cans clinked together, hoping that this time would be the last. And for the longest time, they truly thought it was. A bump in the road. Nothing more. He skipped from job to job, place to place, just in case they were wrong. He'd learned long ago not to form to many bonds, they were too easily broken and people would get hurt when the shattered bits fell. Henry did his best to cope with it all, the death of his wife and the constant fear that his son would be next. He didn't want that- he didn't want to outlive his son. It wasn't right. Shouldn't be like that.
He was 23 when it happened. He found his own little niche, and no matter what that little voice in the back of his head would say, he couldn't leave. By the time he worked up the courage to go, it was to late. The bonds had been made and him leaving would hurt them. He couldn't hurt people. He'd sworn to that the first time, when things had gotten really really bad and his dad had been sitting at his bedside, under the impression that his son was asleep. He could never get the words his dad had uttered out of his head, a light blinking in the back of his head to serve as a constant reminder that it didn't hurt just him. He'd die. They'd be left behind to pick up the pieces. And he couldn't forget that. Not with Henry's broken voice whispering in his head 'I'm not going to be a dad anymore'.
One year later and he relapsed. Gus kept control of Psych, saying Shawn had to take a vacation to anyone who didn't want people to know until they had to. His dad had told him once, in a conversation they'd had trying to understand what the other was going through, of the never ending fear that he might lose him any day, and he'd be alone. He didn't want anyone to go through that, and as it was to late to help his dad or Gus, the best he could do was kept it a secret for as long as possible. It was a quick scare, compared to the others, and a few months later Shawn had waltzed into the police building with a baseball hat on his head and that same stupid grin he'd get when a brilliantly idiotic idea would pop into his head. No one questioned it.
When he and Juliet had begun dating, he contemplated telling her for a long time. But he never did, putting the world before him no matter what the cost. He didn't think he'd be around that long for it to matter, anyway. The headaches had come back.
That had always been the key sign for him. He didn't just get headaches, noise never really effected him. Most would call him lucky. But when he did get headaches they were worse than any other persons by ten fold and they came in clusters, never really stopping because it wasn't just some loud guy screaming on the bus, it was cancer, telling him that it was back.
For a good six months the headaches came and went, before the fuzz came back and it was time for him to admit that once again,life had screwed him over. On one particularly bad day, he wished to get lucky and for this to be the time he died and just let this constant demon that seemed hell bent on ruining his happiness take him away. But that would hurt everyone who stood around his bedside day after day, even though Lassie didn't want to admit it.