They believe that one of the fundamental aspects of the human condition is believing that certain things are right but struggling to live up to it.
They believe that everyone experiences this kind of struggle, but in different ways depending on the person, and that failing at it is inevitable. They believe there is a way for things to be OK even though you fail.
They also believe in an ethical framework where you determine what is right or wrong based not on your own assessment of the pluses and minuses but by following guidance that comes (through scripture, church tradition, etc.) from a higher power whose wisdom is greater than your own.
And they also believe there's some kind of grand plan behind everything, and that the way they fit into this grand plan is to be a priest. They have at some point come to the conclusion that they are supposed to be one, that it is their vocation.
Or at least, within their faith, these beliefs are what is commonly accepted as the right way to look at it. An individual person may or may not actually think or feel exactly like this. Sort of like how one person may become a doctor because they like helping people and someone else may become a doctor because they don't want to disappoint their family.