Sybok talks about pain more than most doctors. In fact, his understanding of emotional suffering—especially that of the repressed variety—is the core of his idiosyncratic belief system.
Here's how he frames it to his first convert at the beginning of the movie:
SYBOK: Each man hides a secret pain. It must be exposed and reckoned with. It must be dragged from the darkness and forced into the light. [...] Share your pain with me and gain strength from it.
Let's be real—this isn't too different from the approach taken by modern therapy. Both examine our relationship with the past in order to help us understand how it shapes our present. By looking at past traumas and better understanding them, we can—as Sybok eloquently states—"gain strength" from our suffering.
Later in the film, Sybok unearths the pain buried in the Enterprise crew's past. He unearths Spock's shame regarding his human heritage. He also explores the devastation McCoy suffered due to his father's terminal illness. In both of these instances, neither man was fully aware of the toll these memories were taking.
So, on the one hand, sure, Sybok seems like a total madman. On the other, though, he makes total sense, and he's good at what he does. What are we supposed to make of that? No matter how you answer, Sybok's ideas about "secret pain" are compelling enough to warrant further examination.