The Thrall was a condition in a minority of members of the Jenchum species.
Jenchum able to generate the Thrall mostly gained this ability during puberty and maintain it through their adult life. The Painter was 15 when he gained the Thrall, and, as of 2367, he'd had the Thrall for 15 years.
During the Thrall, one Jenchum would unconsciously provide positive emotions and feelings to the recipient, boosting their negative emotional state. The recipient, in turn, would give off negative emotions and feelings to the Thrall-enabled Jenchum, completing the cycle.
The two partners in the Thrall usually did not choose one another, and it was not uncommon for Thrall-enabled Jenchum to have one dedicated recipient in their life times. As of 2367, The Painter had only met one person he was Thrall-compatible with. The Thrall could occur between friends, siblings or cousins, married couples, or other types of relationships.
The Thrall was an acceptable part of Jenchum society as it was an evolutionary ability present in some Jenchum. Thrall-enabled Jenchum could not control the ability, although sometimes they were able to control the intensity.
Human anatomy and physiology was not compatible with the Thrall. Exposure to the Thrall could lead to permanent neurological damage or even death. A telepath could ease the separation between a Thrall-enabled Jenchum and human. (Star Trek: The Prospect Chronicles: "Starry, Starry Night")