Star Trek: 10 Crazy Facts About The Trill
8. Trill Symbiosis Commission
Like everything else in life, a bureaucracy governed the whole joining process. This organization was called the Trill Symbiosis Commission, and when it came to symbionts, Trills, and joining, their authority was absolute. If you wanted to go through the process, you had to convince them you were worthy of it.
Prospective hosts went through an entire training and vetting system administrated by the Commission. The vast majority of the applicants were deemed unsuitable and washed out. Even if you completed the training, you could still fail the field test. Jadzia did when Curzon failed her the first time around.
The irony of that situation was Jadzia reapplied, made it through the second time, and actually ended up succeeding Curzon as the host of the Dax symbiont. Though the Commission has never really addressed what possessed the first Trill to join with a symbiont or how they accomplished the medical feat.