Comment by SkyMarshal

Probably a combination of things, but I suspect one reason is, from a relatively young age they make themselves a standard bearer for a belief system and from then on are never allowed the freedom to question it.

They spend their lives attempting to persuade others of the truth of a belief system that many people naturally question as they mature, but priests/ministers have to suppress that tendancy.

It's not until they're on their deathbed do they realize how trapped they were their entire lives, how invested in being trapped they were, and resent it.

Reminds me of the scene at the end of No Coutry For Old Men:

Sheriff Bell: "I always figured when I got older, God would sorta come inta my life somehow.

And he didn't."