Jack first had his Sentinel senses kick in when he was Iran/Iraq. He thought it was because of his fractured skull, but soon found others like him, military men and women with enhanced senses. They underwent intense training to control them, but it never really took. There were drugs to block the senses, which Jack took, but after his son died, he didn’t need them anymore. Then he met Daniel, who made his senses not only light up, but stabilize. Daniel had no idea what he was doing to Jack. He didn’t know how devastated Jack was when he opted to stay behind. Jack did his best to be happy for his friend, but he couldn’t help but think it was fate when Daniel came back to him. As time went on, Jack’s senses became more unstable. Finally, he made a call to a friend, who sent him Blair’s number. Blair and Jim made their way out to the Cheyenne Mountain Complex and were able to diagnose Jack. And Daniel. Daniel was found to be Jack’s Guide, helping him keep his senses in check and use them to help people. Daniel was the focus for Jack so the man didn’t zone out. With this new information, the two men entered into a romantic relationship and helped Blair and Jim find other Sentinels and Guides in the SGC program.