I have an angsty headcanon... that one time (a while before they met Albert), Louis had a very painful heart episode - making him lose consciousness (thought he might die then). He did not tell William however, because the latter wasn't with him when it happened and Louis knows that if he had told him, it would throw his brother off one way or another (taking into account William's worry and under the circumstances they were in before their adoption into the Moriarty household). So he kept it a secret.