Why is Stephen unable to respond to what Father Vincent says? Do we feel that Stephen is at the end enriched by his sorrow?

The Father is telling him that fear always affects those who are afraid; it's always lurking in the background and in one way or another making a difference in how we live our lives.

Sorrow on the hand, changes with the passage of time. Losing something or someone may seem like the end of the world, but our sorrow and sadness will become easier with time....... and sooner or later we can think of the person or thing who made us feel that way and understand why. Sorrow, and our reason for feeling it, can enrich us as people, teach us something, leave us with memories and lessons learned.