Sweets past has been revealed in snippets from Season 4-5. He was adopted as a child and they passed right before he began his job at the Jeffersonian. He gets a kick out of investigating murders and used to be a Metal Head (followed the Black Metal scene). He has scars on his back as if he were whipped, and tries to bond with Booth and Brennan to create a "family-like environment" to make up for the fact he was orphaned as a child, although the crew often jokes about how he is still a baby.
He currently dates Daisy, the loud and talkative intern who was recently given a second chance on the Forensics Team, although none of the team can stand her. He's manipulative and helps Bones and Booth's partnership through regular therapy sessions.